Steps 6.4-6.7B — Local surfaces, reference-exhibit validation, SciFi Observation surface

One commit for the work accumulated in the working tree since Step 6.3,
which had never been split into per-step commits:

- src/local-surfaces.js + src/surface-url.js (new); src/ui.js,
  src/validation.js, src/connection.js and public/index.html updated for
  local-surface hosting and generic surface rendering
- tests: local-surfaces (20), scifi-surfaces (24) and postmessage-interop (7)
  new; connection/museum-gallery/surface-validation suites updated
- reference exhibits: shared/contract-core.js defaults to Contract 5.3
  (major 5, minor 3, xzbt 5.3); museum-gallery advertises its surface
  catalog; aquarium/haunted-house/planetarium adapters updated
- SciFi-XZBT (Step 6.7A/6.7B): surface-mode.js + surface-bus.js,
  Observation-surface boot branch, local-change hooks, view.pillars /
  view.warp-flight targets; fixture byte-identical to G:/.vibe/SciFi-XZBT
- SciFi-XZBT contract adapter handshake fix: the inbound bridge filter no
  longer gates on an exact advisory xzbt value (Contract 5.3 §6.5), only on
  its presence/type, matching the host's own envelope validation; the
  adapter now advertises contract minor 3 / version 5.3.0, which it already
  implemented via the 5.3 surfaces field. Root cause of the five failing
  postmessage-interop tests (host hello was silently dropped).
- docs: architecture 6.4 and 6.7A, reference 6.6 and 6.7; evidence logs;
  test-fixtures/PROVENANCE.md resync record

Test results: NGN 154/154 (was 149/154); postmessage-interop 7/7 (was 2/7);
SciFi contract harness 21/21, real-adapter suite 32/32. git diff --check
clean for changed files; two pre-existing trailing-whitespace lines remain
in test-fixtures/reference-exhibits/scifi/index.html, copied verbatim from
the authoritative SciFi source.

Step 6.7 live verification (browser Observation, packaged standalone) is
still pending and is not claimed here.

README.md

@@ -1,6 +1,7 @@
 # XZBT-NGN Exhibit Engine
 
-A single-exhibit host for XZBT Exhibit Contract 5.2. It negotiates sessions,
+A single-exhibit host for XZBT Exhibit Contract 5.3 (current; see
+[Canonical documentation](#canonical-documentation)). It negotiates sessions,
 discovers descriptor-driven controls, inspects reported state, sends set/invoke
 operations, and displays capabilities, events, revisions, and protocol errors.
 
@@ -9,7 +10,9 @@ Step 5 implementation and verification are recorded in
 descriptors are corrected and both text invocations pass through the operator UI.
 Step 5 is complete: 27 automated tests and all three browser reference checks
 pass. Haunted House reports conformant values, and publishing validation passes.
-Step 6 is planned but has not begun.
+Steps 6.1–6.3 established Contract 5.3 surface discovery and interoperability.
+Step 6.4 adds local presentation panes; see the
+[Step 6.4 report](docs/architecture/XZBT-NGN-Step6.4-Local-Surfaces.md).
 
 ## Run locally
 
@@ -28,6 +31,12 @@ the discovered controls. Enter another path and select **Load exhibit** to switc
   exhibit instance starts with its own initial state.
 - **Disconnect** removes the frame and clears session, catalog, state and events.
 - **Refresh state** reads a new authoritative snapshot.
+- **Presentation surfaces** provides Open, Reload and Close for each local view,
+  with its label, primary status and lifecycle state. URLs resolve against the
+  supplying exhibit's document URL, and must remain same-origin. NGN closes its
+  surface panes when disconnecting, reconnecting the session or switching exhibits.
+  A primary surface already represented by the control frame is marked as open;
+  its lifecycle remains under the Connection controls.
 - Controls are grouped by the exhibit's category (or Other when omitted).
   Reported values and draft inputs are distinct; select **Set** or **Invoke**
   to submit. Rediscovery resets drafts.
@@ -41,6 +50,8 @@ Included reference fixture paths:
 /test-fixtures/reference-exhibits/aquarium/index.html
 /test-fixtures/reference-exhibits/planetarium/index.html
 /test-fixtures/reference-exhibits/haunted-house/index.html
+/test-fixtures/reference-exhibits/museum-gallery/control.html
+/test-fixtures/reference-exhibits/scifi/index.html
 ```
 
 These are locally corrected test/reference fixtures, not NGN product code or
@@ -69,34 +80,55 @@ needed for ordinary operation.
 ## Canonical documentation
 
 This repository is the canonical home for the shared
-[XZBT Exhibit Contract Specification 5.2](docs/contract/XZBT-Exhibit-Contract-Specification-v5.2.md)
-and [XZBT Exhibit Authoring Guide 5.2](docs/authoring/XZBT-Exhibit-Authoring-Guide-v5.2-Step3.7.md).
+[XZBT Exhibit Contract Specification 5.3](docs/contract/XZBT-Exhibit-Contract-Specification-v5.3.md) --
+the current maintained contract, which additively supersedes
+[Contract Specification 5.2](docs/contract/XZBT-Exhibit-Contract-Specification-v5.2.md)
+(retained as a historical predecessor; no 5.2 normative requirement was
+removed, weakened, or reinterpreted) -- and for the
+[XZBT Exhibit Authoring Guide 5.2](docs/authoring/XZBT-Exhibit-Authoring-Guide-v5.2-Step3.7.md).
 The [NGN Implementation Plan 5.2](docs/architecture/XZBT-NGN-Exhibit-Engine-Implementation-Plan-v5.2.md)
 also belongs here. Its historical phase numbering describes the longer roadmap;
 the current Step 5 completion pass does not authorize those later features.
 SciFi-XZBT is an external exhibit implementation, not the NGN architecture.
 `docs/reference/` contains external implementation context, not host requirements.
 
+NGN and its maintained reference exhibits (Museum Gallery, SciFi-XZBT, Haunted
+House, Aquarium, Planetarium) move forward with the contract: there is no
+requirement to keep a currently maintained exhibit, or its test coverage,
+frozen on an old minor version. The `xzbt` envelope field is advisory metadata
+only (Contract §6.5) and is never used as an exact-string compatibility gate;
+real compatibility is negotiated by the handshake and contract fields. A
+genuine, unmaintained Contract 5.2 exhibit remains protocol-compatible with
+this host (Contract §28.2) -- that guarantee is unrelated to which minor
+version the exhibits maintained in *this* repository currently declare.
+
 ## Implementation and MVP boundary
 
 - `src/host.js`: negotiation, requests, discovery, state cache, events and recovery.
 - `src/connection.js`: URL validation and frame load/disconnect/reconnect lifecycle.
+- `src/surface-url.js`, `src/local-surfaces.js`: canonical surface URL resolution
+  and local presentation frame lifecycle, without additional contract sessions.
 - `src/validation.js`: descriptor, state value and argument validation.
 - `src/transport/post-message.js`: same-origin frame transport.
 - `src/ui.js`, `public/`: generic operator interface.
 - `server/serve.js`: loopback static server.
 - `tests/`, `test-fixtures/`: automated checks, reference fixtures and browser evidence.
 
-The MVP supports one exhibit, Contract 5.2 discovery, generic target rendering,
+The MVP supports one exhibit, Contract 5.3 discovery, generic target rendering,
 state inspection, set/invoke, capabilities, events, session/revision/sequence
 tracking, reconnect and synchronization recovery. It has in-memory state only;
 logs retain 200 entries. Idle polling is not enabled. The last connection URL is
-remembered only until the page is reloaded.
+remembered only until the page is reloaded. Contract 5.3 presentation-surface
+discovery and local rendering (Section 31) are implemented and exercised by the
+Museum Gallery reference exhibit; other maintained exhibits do not declare
+multiple surfaces. Surface errors stay within the affected pane. Remote/cast
+display assignment remains deferred. Shared-state synchronization proof remains
+Step 6.5; this step adds no host-side surface communication.
 
-Not implemented: multi-surface presentation, casting/display endpoints, MIDI,
-MCP/webhooks, scenarios/recording, telemetry acquisition, multi-exhibit
-orchestration, persisted operator configurations, remote deployment, accounts,
-databases, cloud services, plugins, commercial licensing or production branding.
+Not implemented: casting/display endpoints, MIDI, MCP/webhooks,
+scenarios/recording, telemetry acquisition, multi-exhibit orchestration,
+persisted operator configurations, remote deployment, accounts, databases,
+cloud services, plugins, commercial licensing or production branding.
 
 ## Publishing records