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>

docs/features/editor-unification/04-part2-single-app-merge.md

@@ -104,7 +104,7 @@ This is the strategic consideration, and it cuts against our current delivery mo
 - **The catch:** that ~190 MB is downloaded by **every** user — even one who only wants the
   calculator. It works *against* the per-tool lazy loading we do now, and a merged binary can't
   be lazily sub-loaded: `wasm-split` / `dlopen` are already assessed as a dead end for us (see
-  [`../perf/bundle-size.md`](../perf/bundle-size.md) and the bundle-composition research). So the
+  [`../perf/README.md`](../perf/README.md) and the bundle-composition research). So the
   tradeoff is "one shared binary, zero duplication, cross-probing possible" vs. "larger first
   load for single-tool users."