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.

test-fixtures/reference-exhibits/scifi/css/style.css

@@ -2428,3 +2428,30 @@ body.xzbt-tv-aspect .observation-overlay:not(.active){display:none}
 @media (max-width:700px){
   .watch-experience-wrap{min-width:150px}.watch-experience-help{font-size:.54rem}
 }
+
+/* ==========================================================================
+   Step 6.7B -- Observation surface mode (body.surface-observation).
+   Applied when this document is booted as the non-authoritative Observation
+   surface (?surface=observation&xi=...). Presentation-only: additive rules,
+   nothing above this block is touched. The console DOM (header/main) is
+   hidden rather than removed -- observation-engine.js keeps binding to it
+   exactly as it does today, unmodified, per the Step 6.7A design.
+   ========================================================================== */
+body.surface-observation > header,
+body.surface-observation > main {
+  display: none !important;
+}
+
+body.surface-observation .observation-overlay {
+  position: fixed;
+  inset: 0;
+  z-index: 1;
+}
+
+/* Exiting/returning to the console is NGN's job (pane lifecycle), not the
+   surface's -- these controls would otherwise mutate the *console's* own
+   view.observation overlay flag from a stray click (report SS7/SS8). */
+body.surface-observation #obs-btn-exit,
+body.surface-observation #observation-return-pill {
+  display: none !important;
+}