docs(visual): specify the phase 4a scene, primitive, and appearance contract

Open the Visual subsystem contract at Format Specification revision 0.5 with
section 17, covering PRD 69-76: the renderer-neutral pipeline, canonical units
on a single degrees convention, the visuals container, layers, the scene model
with three coordinate spaces and three fit modes, 2.5D depth with a fixed sign,
stable sorting, parallax, perspective factor and per-object fog, the fourteen
primitives of Visual Primitive Set 0.1, common visual properties, a normative
transform composition order, appearance with the eight-mode safe blend set and
gradient paint, paths and splines with independently addressable points, text
restricted to the three generic font families, and the once-at-instantiation
resolution boundary for visual ValueSpec fields.

Visual objects are keyed rather than listed and carry no id field, matching the
audio node-identity rule that a Phase 3a blocking defect established. No visual
property becomes externally addressable in this slice; a binding, set, or
override naming one remains ERR_UNSUPPORTED_TARGET, and section 8.1 is
unchanged until section 19.1 adds its rows.

Add five diagnostic codes to the section 7 table, record the eight-slice Phase 4
delivery plan, mark the section 11 Visuals register row in progress, and refresh
the implementation status, which had not been updated since the 3c-1 contract.

Documentation only; no runtime, schema, fixture, or test file is touched.
npm test passes 102 tests with zero failures.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01GwmTQU6zLwh3qbBvq5zf5B

docs/XZBT_0-1_Implementation_Plan.md

@@ -35,6 +35,27 @@ Slices 3c-1 through 3c-3 can be implemented and verified without an audio device
 
 An installment is complete only when its own contract, implementation, tests, and evidence record are all in place. A contract installment is reviewed against its PRD sections and the shared contracts before its dependent installment begins; the Phase 3a review, which closed four blocking defects before Phase 3b started, is the worked example.
 
+## Phase 4 delivery slices
+
+Phase 4 is the largest milestone in this plan: twenty-one PRD sections (69-89) plus the shared ceilings of PRD 119-120, a fourteen-item acceptance challenge (PRD 130), and the first visuals for four reference exhibits. It is delivered in eight slices under the same rule as Phase 3c — each slice ends green, committed, and pushable on its own — and under the project rule that a subsystem contract is finished immediately before the implementation that depends on it.
+
+| Slice | Content | Depends on | Blocked by |
+| --- | --- | --- | --- |
+| 4a — Contract I | Format Specification section 17: pipeline and canonical units, the `visuals` container, layers, scene model and coordinate/fit modes (PRD 70), 2.5D depth (PRD 71), the fourteen geometry primitives (PRD 72), common visual properties (PRD 73), the transform model (PRD 74), appearance and the safe blend set (PRD 75), and paths and splines (PRD 76). Documentation only; no runtime change. | Sections 1-13 | — |
+| 4b — Contract II | Format Specification section 18: visual components (PRD 77), particle systems (PRD 78), placement distributions (PRD 79), emitters (PRD 80), repeaters (PRD 81), the behavior vocabulary (PRD 82), procedural fields (PRD 83), and trails, ribbons, and links (PRD 84). Documentation only. | 4a | — |
+| 4c — Contract III | Format Specification section 19: visual automation and loop modes (PRD 85), visual lifecycle and ownership (PRD 86), camera and projection (PRD 87), post-processing (PRD 88), visual safety limits (PRD 89), the centralized runtime ceilings of PRD 120, and the visual rows added to the section 8.1 target-capability table. Documentation only. | 4a, 4b | — |
+| 4d — Renderer core | Canvas 2D backend, the scene/coordinate/fit resolution, layers, the transform stack, the fourteen primitives, appearance and paint, paths and splines, and depth sorting. Widest new-code surface in the phase. | 4a | — |
+| 4e — Procedural systems | Visual components, particles, distributions, emitters, repeaters, behaviors, fields, trails/ribbons, and distance links. | 4b, 4d | — |
+| 4f — Automation, lifecycle, and effects | Visual automation into the shared section 8 resolution path, spawned-system lifecycle and ownership, camera projection, post-effect passes, and enforcement of the centralized ceilings. Touches the Phase 2 and Phase 3c resolution path, so it carries the widest regression surface in Phase 4. | 4c, 4e | — |
+| 4g — Challenge fixtures and exhibit visuals | The fourteen PRD 130 challenge fixtures, proven to need no subject-specific renderer code, and the first visuals for Exhibits A-D. | 4d, 4e, 4f | — |
+| 4h — Early combined GC6 benchmark | Freeze the benchmark ID, version, seed, and counts; run the 30-second warm-up and 120-second measurement window; record frame intervals, runtime counters, and memory samples; derive the workload ceilings that fix later optimization strategy. | 4g | **User-observed.** Requires a real reference computer and browser at 1920 x 1080; cannot be closed by automated tests. |
+
+Slices 4a through 4g are implementable and verifiable without a display measurement. Slice 4h and the PRD 130 visual acceptance challenge are not: the challenge's judgement is visual, and the benchmark is a hardware measurement, which the recorded test boundary assigns to user-performed runs. Phase 4 is not accepted until 4h and the visual challenge close, regardless of the state of the first seven slices.
+
+The three contract slices land before any of the implementation slices, because the section 17-19 contract fixes decisions that 4d through 4f would otherwise have to relitigate: the resolution boundary for visual ValueSpec fields, the transform composition order, depth semantics, and which visual properties are externally addressable at all. Whether the finished contract also passes a multi-model review before 4d begins — the pass that found thirteen defects in sections 14-16, nine of which any single reviewer would have missed — is an open decision recorded here rather than assumed.
+
+Phase 3 remains unaccepted while its user-observed gates (traces 15-17 of section 16.11 and the Phase 1 direct-file restart observation) are open. Those gates are pending measurement, not failed, and no Phase 4 slice depends on them; Phase 4 nonetheless carries that debt forward and cannot close it.
+
 ## MVP completion-criteria map
 
 | Completion criterion group | Primary milestone | Final evidence |