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.

AGENTS.md

@@ -55,10 +55,23 @@ Authoring Guide, and owns the NGN Implementation Plan. Documents in
 Reference exhibits under `test-fixtures/` are test/reference fixtures, not product
 code. Keep exhibit-specific names and paths in test configuration or documentation.
 
-- XZBT Exhibit Contract Specification v5.2
+XZBT-NGN and its maintained reference exhibits move forward with the contract.
+Contract 5.3 is the current maintained contract; it supersedes 5.2 additively
+(no 5.2 normative requirement was removed, weakened, or reinterpreted). The
+`xzbt` envelope field is advisory metadata only (Contract §6.5) and MUST NOT be
+used as an exact-string compatibility gate anywhere in this repository --
+real compatibility is negotiated by the handshake and contract fields.
+Backward compatibility with a genuine 5.2 exhibit remains a protocol-level
+requirement of the contract itself (Section 28.2), but that is a different
+thing from keeping a *currently maintained* reference exhibit or its test
+coverage frozen on 5.2: there is no such requirement, and maintained exhibits
+are expected to advance with the contract instead.
+
+- XZBT Exhibit Contract Specification v5.3 (current; supersedes v5.2 below)
 - XZBT-NGN Exhibit Engine Implementation Plan v5.2
 - XZBT Exhibit Authoring Guide v5.2
 - Step 3.7 reference exhibit report
+- XZBT Exhibit Contract Specification v5.2 (historical predecessor, retained for context)
 
 - Do not infer XZBT-NGN behavior from SciFi-XZBT source code.
 - SciFi-XZBT is an external exhibit implementation, not the NGN architecture.