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

tests/README.md

@@ -24,7 +24,7 @@ npm test          # setup:kicad + the full merged run (same projects as CI)
 
 One merged config (`playwright.config.ts`) drives every wasm suite as
 Playwright *projects*; `npm run test:e2e` runs the CI set: `wx-chromium`,
-`kicad-firefox`, `kicad-chromium`, `asyncify-firefox`, `coroutine-firefox`.
+`kicad-firefox`, `kicad-chromium`, `jspi-firefox`, `coroutine-firefox`.
 The KiCad specs (heavier — they need the docker-built KiCad WASM) run on BOTH
 engines; `npm run test:kicad` is the firefox-only shortcut. The React web app
 suite is separate: `npm run test:web` (see `playwright-web.config.ts`).
@@ -59,7 +59,7 @@ tests/
 ├── apps/              # Built WASM test applications
 │   ├── minimal_test.html  # Main test app
 │   └── standalone/        # Individual component test apps
-├── playwright.config.ts       # THE merged config (wx / kicad / asyncify / coroutine / perf projects)
+├── playwright.config.ts       # THE merged config (wx / kicad / jspi / coroutine / perf projects)
 └── playwright-web.config.ts   # React web-app suite (own server stack)
 ```
 
@@ -304,13 +304,12 @@ Button positions (relative to canvas):
 ## Open tasks
 
 - ~~Research: are the Asyncify fiber shims still needed under native-EH?~~
-  **Resolved at doc 20 D-1** (legacy-runtime deletion): the ablation builds
-  (`races_test_noheal` / `races_test_nosleepfix`) and their shim-redundancy pins in
-  `asyncify/asyncify-races.spec.ts` pinned a runtime that no longer exists — the
-  scheduler shim (`scripts/common/shims/asyncify-scheduler.js`) is the only runtime
-  and subsumes the handleSleep save/restore; the fiber trampoline self-heal (§3c)
-  remains injected unconditionally. The green battery runs every scenario against
-  the scheduler glue.
+  **Resolved at doc 20 D-1, then mooted by the JSPI migration (2026-08)**: the
+  ablation builds (`races_test_noheal` / `races_test_nosleepfix`) and their
+  shim-redundancy pins (in the since-deleted `asyncify/asyncify-races.spec.ts`)
+  pinned a runtime that no longer exists, and the asyncify scheduler shim they
+  were measured against retired with the backend. The semantic race battery
+  lives on in `jspi/suspend-races.spec.ts` against the JSPI runtime.
 
 ## Collab e2e — legacy vs v2 bundles, and repro markers