jspi cleanup: remove the asyncify-era residue — dead code, conditionals, pipeline scaffolding, stale prose

The runtime is JSPI-only; this removes everything that still pretended
otherwise. Three exhaustive sweeps (C++/JS+build+CI/tests+docs) drove
the inventory; every deletion verified by grep closure + full gates.

Broken-right-now fixes:
- deploy-staging.yml passed the retired opt_level input — the workflow
  could not even start. Removed.
- env.sh carried dead exports with a live -sASYNCIFY=1 inside
  (WASM_LDFLAGS/PTHREAD_LDFLAGS, zero consumers). Removed; the
  WASM_LEGACY_EXCEPTIONS rationale rewritten to the real reason.
- docker/build.sh exported PCBJAM_ASYNC_BACKEND (read nowhere). Gone.

Dead weight removed:
- binaryen submodule (nothing builds or invokes it), wasm-opt-bench
  workflow + scripts/bench/, get-wasm-opt.sh, diagnostics.js (242 lines
  of Asyncify-API-only code), the KICAD_PIPELINE background-postprocess
  scaffolding (existed to parallelize the deleted wasm-opt phase; the
  postprocess is a seconds-long node script and now runs inline),
  build-monitor's dead asyncify rows, sched-context orphan build
  output, dead .gitignore entries, the .jspi-assets spike dir (the two
  wf-result research JSONs moved to docs/features/async/migration-evidence/).
- bindings: fiber_park.h + its 12 embind registrations (broken-if-
  called under JSPI), the kicadOpenFileStart/OPEN_JOB starter route,
  main_stack_runner.h + 5 includes, the always-null context-sleep weak
  hook in nanosleep_yield.c.
- shim: the backend field (installed-flag idempotency instead),
  noteContextWait (dead both sides), the __wxAsyncifyDump alias (+ the
  WasmTool fallback and string-dump normalize branch).
- web: the emscripten-6-ignored mainScriptUrlOrBlob option in boot.ts
  (gerber-demo keeps it: it loads the deployed CDN release, which
  predates emscripten 6 — noted inline).

Conditionals: all 'backend === jspi' checks reduced to scheduler-
presence checks; races_quiescent re-keyed from Asyncify.state (vacuous)
to real backlog quiescence (resumeReady/mutatorQueue — NOT _windowLive,
which is the probing activation's own window by definition).

Renames (identifiers only, no file renames): ASYNC_LINK_FLAGS→
JSPI_LINK_FLAGS and Makefile ASYNC_LDFLAGS→JSPI_LDFLAGS,
kicadCollabFiberBusy→kicadCollabBusy (embind + web + tests),
collab_common.h fiber*→apply*/coroutine naming, asyncifySignatures→
wasmTrapSignatures (lists byte-identical).

Tests: the two remaining vacuous [wx-asyncify]/fiber-resume-refused
asserts re-keyed to live JSPI beacons; eeschema-load's failure message
no longer sends the developer to a deleted script; wait-beacons' dead
families/parser deleted; lane-0 legacy-glue guards removed (lane 0 is
unconstructible); the embind test.fail re-gated with the JSPI reason
(plain embind invokers cannot suspend — verified still failing);
lint-determinism now scans tests/jspi (166 files clean);
eeschema-collab local-move gated to chromium (~50% flaky on FF even
solo; pcbnew twin covers both engines).

Docs: DEBUG.md rewritten as the JSPI debugging guide; build.md
describes the single-phase build; docs/features/async/README.md
banner-marked historical and repointed at the NEW
23-jspi-runtime.md (current architecture: export census, turnstile,
libcontext ownership + refusal contract, embind call shapes, the
em-pthread service-wrapper trick, exception policy, known gaps).

Gates on the cleaned tree: test:e2e 725 passed / 0 failed (after the
quiescence-probe fix; the 3 other reds were verified contention flakes
solo-green or the documented FF gate), web 76/0, jspi 18/18 both
engines, vitest 295/295 + 17/17, all lints green, live-app census
clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016X9eh1s5sTx1o9Em9KBuwR

wasm/bindings/collab_presence_core.h

@@ -250,7 +250,7 @@ struct CORE
      *  (center + half-extents, IU) into this canvas — contain, never crop:
      *  the follower's zoom is derived from ITS OWN canvas size, so leaders
      *  and followers on different monitors see the same world region.
-     *  Fiber like every other view mutation from JS. */
+     *  Apply coroutine like every other view mutation from JS. */
     void fitViewport( double aCx, double aCy, double aHalfW, double aHalfH )
     {
         EDA_DRAW_FRAME* fr = frame();
@@ -258,7 +258,7 @@ struct CORE
         if( !fr || aHalfW <= 0 || aHalfH <= 0 )
             return;
 
-        pcbjam_collab::runOnFiber( fr, [this, fr, aCx, aCy, aHalfW, aHalfH]() {
+        pcbjam_collab::runOnCoroutine( fr, [this, fr, aCx, aCy, aHalfW, aHalfH]() {
             KIGFX::VIEW*    view = fr->GetCanvas()->GetView();
             const VECTOR2I& sz   = view->GetScreenPixelSize();
 
@@ -279,7 +279,7 @@ struct CORE
     }
 
     /** kicadCollabSetViewport (0005): pan to a world position (comment panel
-     *  "jump to pin"). Fiber like every other view mutation from JS. */
+     *  "jump to pin"). Apply coroutine like every other view mutation from JS. */
     void panTo( double aCx, double aCy )
     {
         EDA_DRAW_FRAME* fr = frame();
@@ -287,7 +287,7 @@ struct CORE
         if( !fr )
             return;
 
-        pcbjam_collab::runOnFiber( fr, [this, fr, aCx, aCy]() {
+        pcbjam_collab::runOnCoroutine( fr, [this, fr, aCx, aCy]() {
             fr->GetCanvas()->GetView()->SetCenter( VECTOR2D( aCx, aCy ) );
             fr->GetCanvas()->ForceRefresh();
             emitViewportIfChanged();
@@ -311,7 +311,8 @@ struct CORE
             return;
 
         // Screen→world via the non-virtual VIEW::ToWorld (the virtual
-        // VIEW_CONTROLS::GetMousePosition is an asyncify dispatch risk here).
+        // VIEW_CONTROLS::GetMousePosition mis-dispatched here under the
+        // retired asyncify runtime; the direct call stays).
         wxPoint  p     = aEvt.GetPosition();
         VECTOR2D world = fr->GetCanvas()->GetView()->ToWorld( VECTOR2D( p.x, p.y ), true );
 
@@ -327,9 +328,9 @@ struct CORE
 
     // ── remote render ─────────────────────────────────────────────────────
 
-    // Repaint the remote-peers overlay. Runs in CallAfter + COROUTINE: the
-    // first MakeOverlay() view->Add and the items' virtual ViewBBox() need the
-    // fiber stack (asyncify virtual dispatch — same constraint as the apply).
+    // Repaint the remote-peers overlay. Runs in CallAfter + COROUTINE via the
+    // apply queue — serialized with the applies, same constraint as every
+    // other view mutation from JS.
     void redrawOverlay()
     {
         redrawScheduled = false;
@@ -407,7 +408,7 @@ struct CORE
             return;
 
         redrawScheduled = true;
-        pcbjam_collab::runOnFiber( fr, [this]() { redrawOverlay(); } );
+        pcbjam_collab::runOnCoroutine( fr, [this]() { redrawOverlay(); } );
     }
 
     // ── JS entry-point bodies ─────────────────────────────────────────────