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.

docs/contract/XZBT-Exhibit-Contract-Specification-v5.2.md

@@ -1,6 +1,13 @@
 # XZBT Exhibit Contract Specification
 ## Version 5.2
 
+> **Historical:** Superseded, additively, by
+> [Contract Specification 5.3](XZBT-Exhibit-Contract-Specification-v5.3.md),
+> the current maintained contract. No normative requirement in this document
+> was removed, weakened, or reinterpreted by 5.3; it is retained here for
+> historical context, not as a second currently-maintained contract version.
+> XZBT-NGN and its maintained reference exhibits target Contract 5.3.
+
 **Status:** Proposed normative specification  
 **Document version:** 5.2  
 **Contract family:** XZBT Exhibit Contract