docs(visual): specify the phase 4b component, procedural system, and behavior contract

Format Specification section 18 (PRD 77-84) closes slice 4b: visual
components and their inputs scope, particle systems with a normative
integrator and life ramps, the nine placement distributions, emitters
with exact fractional-accumulator emission timing, repeaters and the
copy-scoped repeat.* namespace, the seventeen-behavior vocabulary and
its channel set, the six procedural fields with a normative coherent
noise function, and trails, ribbons, and links.

Seven diagnostic codes are added to the section 7 table. Section 17 is
amended in four places the new section requires: the visuals.fields
container, the component visual object type beside the closed fourteen
primitives, the now-declared behaviors field, and the widened visual
limit row. Revision 0.5 -> 0.6.

Documentation only; no runtime, schema, fixture, or test file changed.
npm test remains 102 passing.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_019dTNHWyTdFobdevrKspoV6

docs/evidence/phase4/2026-09-06-phase4b-contract.md

@@ -0,0 +1,160 @@
+# Phase 4b evidence — Visual subsystem contract, part II (Format Specification section 18)
+
+**Date:** September 6, 2026
+**Slice:** 4b — Contract II (documentation only; no runtime change)
+**Baseline:** `main` at `3e27a66` (Phase 4a, section 17)
+**Specification revision:** 0.5 → 0.6
+**PRD coverage:** 77-84
+
+## What this slice fixes
+
+Section 18 continues the Visual contract section 17 opened. It covers reusable visual components and
+their input scope, particle systems and the normative motion integrator, the nine placement
+distributions, emitters and exact emission timing, repeaters and the copy-scoped `repeat.*` namespace,
+the seventeen-behavior vocabulary and the channel set behaviors may write, the six procedural fields and
+the normative coherent-noise function behind them, and trails, ribbons, and links.
+
+Section 19 (Phase 4c) remains the owner of visual automation, lifecycle and ownership, camera,
+post-processing, and the centralized ceilings. The forward references to 19.1, 19.2, and 19.5 are the
+only unresolved numeric references in section 18, and they close when 4c lands.
+
+## Decisions taken, so 4e does not relitigate them
+
+1. **`component` is a visual object type, not a fifteenth primitive.** Visual Primitive Set 0.1 stays
+   closed at the fourteen geometry primitives section 17.9 fixed. Section 18.1 adds `component` beside
+   that set, exactly as 15.11 added a `component` node beside the audio node set 14.3 opened, so a
+   component instance can be placed inline in any `content` or `children` container instead of only
+   being reachable through a repeater of count one.
+2. **Visual component parameters admit four types, audio's admit one.** `number`, `boolean`, `string`,
+   and `color` — the shared scalar types of section 2. The audio contract (15.15) permits only `number`
+   because an audio node field is always numeric; a visual field may be a color, a label, or a
+   visibility flag. `min`, `max`, and `unit` remain number-only.
+3. **`inputs.*` and `repeat.*` are construct-scoped, not document namespaces.** Both follow the rule
+   15.15 fixed for audio components: legal only inside their construct, `ERR_INVALID_REFERENCE`
+   anywhere else, absent from the section 1.3 namespace list, and granting no section 8.1 capability.
+4. **A component's expansion path, not its ID, is its stable instance key** for section 9.3 stream
+   derivation. Two instances of one component therefore sample independently, and reordering unrelated
+   siblings does not perturb either — the same property 14.12 gives audio components.
+5. **Particle motion has a normative integrator.** Semi-implicit Euler, velocity before position, with
+   drag as `(1 - drag)^dt` so it is tick-length independent. A renderer that integrates position first,
+   or applies drag once per tick regardless of `dt`, produces visibly different motion at a different
+   tick rate and does not conform. Traces 5 and 6 pin both halves.
+6. **Life ramps interpolate resolved endpoints; they do not re-sample.** `size`, `opacity`, and `color`
+   may ramp over normalized age using the same four curves PRD 85 fixes for automation. `from` and `to`
+   resolve once at particle creation, so 17.14 and 9.3 hold unchanged: rendering still consumes no
+   procedural stream. This is what gives PRD 78 its `age` property without breaking reproducibility.
+7. **Emission timing is a fractional accumulator, not a per-tick rounding.** Cumulative count after `t`
+   seconds at a constant rate is `floor(rate * t)` exactly, under any tick length, with no drift and no
+   reset when automation changes the rate later.
+8. **Unbounded emission fails validation.** A system that declares emission but neither a per-item
+   `lifetime` nor a total `limit` is `ERR_UNBOUNDED_EMISSION`. An initial `count` with no `lifetime`
+   stays legal — a fixed starfield that never expires is the common case, not the dangerous one.
+9. **Pool exhaustion evicts oldest-first**, mirroring the voice eviction of 16.6, and is silent at the
+   system level. The aggregate ceiling across systems and its diagnostic belong to 19.5.
+10. **Index-driven placement needs a known count.** `even` modes and the `grid` type place item `i` of
+    `n`, so they are legal in a repeater and in a fixed `count` or `burst` population and are
+    `ERR_INVALID_DISTRIBUTION` on continuous `rate` emission. Distribution sample-consumption order is
+    normative (`x`, then `y`, then `z`; angle before radius) so fixtures place identically everywhere.
+11. **Coherent noise is specified, not left to the renderer.** Three-dimensional gradient noise on the
+    integer lattice, twelve edge-midpoint gradients, a `256`-entry permutation shuffled from the field's
+    own stream at instantiation, and the quintic fade `6u^5 - 15u^4 + 10u^3`. Reproducibility (9.3)
+    promises identical decisions rather than identical pixels, but a field that drove motion differently
+    on two conforming renderers would make a fixture untestable, so the function itself is pinned.
+    `curl` is the default mode because it is divergence-free and will not pile items into a point.
+12. **Ribbons are a trail render mode, not a separate system type.** A ribbon is the same history buffer
+    drawn with area instead of a stroke, so `trail.mode` is `line`, `ribbon`, or `points`. PRD 84 is
+    satisfied without a fourth procedural system.
+13. **`links` is illegal on an emitter.** A `particles` system and a `repeater` have bounded, known
+    populations; an emitter's changes continuously and its pair search would have to be rebuilt every
+    tick at exactly the cost the ceiling exists to prevent. All three PRD 84 rules — `distance`,
+    `nearest`, `index` — are specified, with `distance` the required 0.1 minimum.
+14. **Behaviors write internal channels; they do not extend section 8.1.** The `property` channel set of
+    18.6 is a behavior's own permitted write set. External addressability of visual properties remains a
+    section 19.1 decision, and a binding, `set`, or `override` naming anything introduced here is still
+    `ERR_UNSUPPORTED_TARGET`.
+15. **The morph rule section 17.13 deferred is now fixed:** equal `type`, equal point count, and equal
+    spline `mode`, with `ERR_MORPH_INCOMPATIBLE` on any mismatch. Resampling one shape onto another's
+    point count would change the geometry the author wrote, so it fails validation instead.
+
+## Amendments to section 17
+
+Four, all consequences of this slice rather than corrections:
+
+- **17.3** gains a `fields` row for the `visuals.fields` container. Without it the strict unknown-field
+  policy would make every declared field `ERR_UNKNOWN_FIELD`.
+- **17.9** records that Visual Primitive Set 0.1 is closed at fourteen and that 18.1 adds the
+  `component` object type beside it.
+- **17.10**'s `behaviors` row no longer reads "Rejected as `ERR_UNKNOWN_FIELD` until section 18 declares
+  them"; it now carries the `0` to `8` bound.
+- **17.15**'s local `ERR_VISUAL_LIMIT_EXCEEDED` row names the procedural-system ceilings this section
+  adds.
+
+No decision recorded in section 17 was reversed.
+
+## New diagnostic codes
+
+Seven were added to the section 7 table, which remains the single authoritative list:
+`ERR_INVALID_DISTRIBUTION_TYPE`, `ERR_INVALID_DISTRIBUTION`, `ERR_INVALID_BEHAVIOR_TYPE`,
+`ERR_INVALID_BEHAVIOR_TARGET`, `ERR_INVALID_FIELD_TYPE`, `ERR_MORPH_INCOMPATIBLE`, and
+`ERR_UNBOUNDED_EMISSION`. Section 18.9 repeats them for locality, exactly as 15.18, 16.10, and 17.15 do.
+
+`ERR_COMPONENT_RECURSION` is reused for visual component recursion and over-nesting rather than
+duplicated under a visual name, along with `ERR_VISUAL_LIMIT_EXCEEDED`, `ERR_INVALID_REFERENCE`,
+`ERR_INVALID_RANGE_ORDER`, `ERR_OUT_OF_BOUNDS`, `ERR_TYPE_MISMATCH`, `ERR_UNKNOWN_FIELD`,
+`ERR_SCHEMA_VALIDATION`, and `WARN_AUTOMATION_FALLBACK`.
+
+## Authoring limits fixed in this slice
+
+| Limit | Bound | Diagnostic |
+| --- | --- | --- |
+| Visual component nesting depth | 8 (matches audio, 15.14) | `ERR_COMPONENT_RECURSION` |
+| Particle `capacity` per system | 4096 | `ERR_VISUAL_LIMIT_EXCEEDED` |
+| Emitter `capacity` per system | 512 | `ERR_VISUAL_LIMIT_EXCEEDED` |
+| Repeater `count` | 1024 | `ERR_VISUAL_LIMIT_EXCEEDED` |
+| `burst` entries per system | 16 | `ERR_SCHEMA_VALIDATION` |
+| Behaviors per object | 8 | `ERR_VISUAL_LIMIT_EXCEEDED` |
+| Declared fields per exhibit | 8 | `ERR_VISUAL_LIMIT_EXCEEDED` |
+| Fields referenced per system or behavior | 4 | `ERR_VISUAL_LIMIT_EXCEEDED` |
+| Noise `octaves` | 4 | `ERR_OUT_OF_BOUNDS` |
+| Grid `columns` / `rows` | 256 each | `ERR_VISUAL_LIMIT_EXCEEDED` |
+| Trail `length` | 128 samples | `ERR_VISUAL_LIMIT_EXCEEDED` |
+| Links per system per tick (`maxLinks`) | 1024 | `ERR_VISUAL_LIMIT_EXCEEDED` |
+| Statically known population under a pairwise link rule | 256 | `ERR_VISUAL_LIMIT_EXCEEDED` |
+| Nearest-neighbour links per item (`count`) | 8 | `ERR_SCHEMA_VALIDATION` |
+
+These are authoring bounds a document must satisfy at validation. The aggregate runtime ceilings —
+total active particles, total spawned systems, total links, total field evaluations, and the per-tick
+budget they share (PRD 89, 120) — remain slice 4c and are not set here. Slice 4h is what determines
+whether these bounds are the right ones.
+
+## Verification performed
+
+This slice changes documentation only. No runtime source, schema, fixture, or test file was touched.
+
+- Every diagnostic code used in section 18 (19 distinct codes) resolves to a row in the section 7 table;
+  zero unresolved, checked programmatically.
+- Every numeric cross-reference in section 18 resolves to an existing heading, except the intended
+  forward references to 19.1, 19.2, and 19.5.
+- All code fences in section 18 are balanced and every table row is well formed.
+- `npm test` passes 102 tests with zero failures, confirming the existing suite is unaffected.
+- The section 11 contract register's Visuals row now reads **In progress (Rev 0.6 / Phase 4a-4b)** and
+  names what remains for 4c.
+
+## Test boundary
+
+Traces 1-19 of section 18.10 are automated and belong to slice 4e, where the procedural systems exist to
+run them. Traces 20 and 21 — the PRD 130 visual challenge judged on a real display, and the early
+combined GC6 benchmark at `1920 x 1080` — are user-observed and close slice 4h. Trace 20 is the same gate
+as trace 15 of 17.16, restated for locality rather than added as a second gate.
+
+## State at this checkpoint
+
+Sections 17 and 18 are contract only. No visual runtime code exists: `src/runtime` has no renderer
+module, the JSON Schema has no visual definitions, `exhibits/` has no visual fixture, and nothing has
+been drawn from any build. Phase 3 also remains unaccepted while its own user-observed gates (traces
+15-17 of section 16.11) and the Phase 1 direct-file restart observation are open; no Phase 4 slice
+depends on them, and no Phase 4 slice can close them.
+
+Whether sections 17-19 pass a multi-model review before slice 4d begins — the pass that found thirteen
+defects in sections 14-16, nine of which any single reviewer would have missed — remains an open
+decision, and section 18 has now roughly doubled the surface that review would cover.