docs+test(wasm): Part 2 as-built record; cross-face probe spec

- docs/features/editor-unification/06-part2-implementation.md — the merged
  kicad_editor as-built record: symbol-collision surgery (incl. the 21-symbol ODR
  audit), embind dispatcher, wiring, and validation results (frame-runtime 4/4,
  full suite 67 passed / 1 known pre-existing 3d-viewer drag failure fixed on main
  by 7630c7e, collab 10/10, cross-face probe green). README status/index updated;
  dangling perf/bundle-size.md links → perf/README.md.
- tests/kicad/xface-probe.spec.ts — permanent cross-face guard: a --frame=sch
  session opens Preferences and asserts the PCB engine's pages ("PCB Editor",
  "Footprint Editor") exist — i.e. KiFACE(FACE_PCB) lazy-starts inside a schematic
  session (absent on single-kiface bundles). Routed to BIG_MODULE_SPECS.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/playwright-kicad.config.ts

@@ -109,6 +109,8 @@ const BIG_MODULE_SPECS = [
   "**/eeschema-ui.spec.ts",
   "**/eeschema-url-regex.spec.ts",
   "**/symbol_editor.spec.ts",
+  // Cross-face probe: schematic session lazily starts the PCB kiface (Preferences).
+  "**/xface-probe.spec.ts",
 ];
 
 // Runtime-perf specs run ONLY on the Chromium 'perf' project below: they need