Step 6.3 Complete — Generic Contract 5.3 Surface Discovery

- src/validation.js: add validateSurfaceCatalog() implementing Contract
  5.3 §§31.2-31.5 normative validation order (validate individuals ->
  discard invalids -> evaluate primary invariant against working set)
- src/host.js: emit xzbt:5.3 advisory (§6.5); store contractMinor;
  add surfaces=[] to resetView(); parse surfaces in refresh(true)
- src/ui.js: add buildSurfaces() for generic descriptor-driven surface
  display; integrate renderedSurfaces tracking into render()
- public/index.html: subtitle updated to Contract 5.2/5.3; add
  surfaces-section with surfaces div
- public/style.css: add .surface, .surface-badge, .surface-meta styles
- tests/surface-validation.test.js: 45 new tests covering all Part 8
  malformed surface cases, lifecycle, registry refresh, 5.2 regression
- docs/reference/XZBT-NGN-Step6.3-Surface-Discovery-Verification.md

Test results: 88/88 pass (43 pre-existing + 45 new)
Genericity scan: zero matches in src/, public/, server/
git diff --check: clean

public/style.css

@@ -24,3 +24,9 @@ button:focus-visible, input:focus-visible, select:focus-visible { outline: 2px s
 .target { padding: 16px 12px; }
 .target code { overflow-wrap: anywhere; }
 @media (max-width: 600px) { header, main { padding: 12px; } section { padding: 12px; } .connection-row input { flex-basis: 100%; } }
+.surface { border-top: 1px solid #354658; padding: 14px 12px; }
+.surface h3 { margin: 0 0 2px; font-size: 15px; }
+.surface code { display: block; font-size: 12px; color: #7eb8d8; overflow-wrap: anywhere; margin-bottom: 4px; }
+.surface-badge { display: inline-block; font-size: 11px; padding: 2px 7px; border-radius: 3px; background: #233444; margin-right: 6px; }
+.surface-badge.primary { background: #1a4a2e; color: #80d6ad; }
+.surface-meta { font-size: 13px; color: #b7c4d1; margin: 4px 0 0; }