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/timer_park.h

@@ -1,27 +1,24 @@
 /*
- * Test-only deterministic repro lever for the production board-load trap
- * family ("index out of bounds" / "unreachable executed" in doRewind —
- * gal-refresh-timer investigation, docs/features/async/14-open-settle-gate.md
- * lineage).
+ * Test-only deterministic lever: a SUSPENDING wx-timer handler running
+ * concurrently with the main loop's per-frame suspension (gal-refresh-timer
+ * investigation, docs/features/async/14-open-settle-gate.md lineage).
  *
- * The surviving hypothesis after the v0.1.19 crash log (2026-07-31): a wx
- * timer callback is a FRESH JS→wasm entry (emscripten_async_call →
- * TimerCallbackFunc::Run → Notify). The main loop is Asyncify-parked in
- * wxWasmYieldToBrowser for most of wall-clock time, so a timer handler that
- * itself parks creates TWO live Asyncify contexts over the single-slot
- * Asyncify.currData — the emscripten #9153 family the handlesleep.js shim
- * silently repairs. Add a fiber swap (the collab entries run on
- * TOOL_MANAGER coroutines — emscripten_fiber_swap, which bypasses the shim's
- * allocateData accounting entirely) and the prod trap's exact second stack
- * (doRewind → finishContextSwitch under __asyncjs__wxWasmYieldToBrowser)
- * becomes constructible on demand.
+ * A wx timer callback is a FRESH JS→wasm entry (emscripten_async_call →
+ * TimerCallbackFunc::Run → Notify). The main loop spends most wall-clock time
+ * suspended in wxWasmYieldToBrowser, so a timer handler that itself suspends
+ * puts a second suspended activation in flight over the shared scheduler
+ * state — the concurrency shape the scheduler's turnstile and the dispatch
+ * interlock must absorb. While the handler is suspended,
+ * wxWasmDispatchParked() reads true and every other due timer spins the 17 ms
+ * retry loop — the [wx-timer] storm diagnostic window this lever opens on
+ * demand.
  *
- * Natural occurrences need a timer handler that parks mid-paint (GAL init /
- * lib bridge) — scheduler-dependent, never reproduced locally in 10+
+ * Natural occurrences need a timer handler that suspends mid-paint (GAL init
+ * / lib bridge) — scheduler-dependent, never reproduced locally in 10+
  * attempts. This lever makes the window deterministic: arm a one-shot wx
  * timer whose Notify() emscripten_sleep()s for a fixed time; the e2e then
- * hammers fiber-based collab entries through the window and asserts the
- * runtime SURVIVES (red on a build where the hypothesis holds).
+ * hammers coroutine-based collab entries through the window and asserts the
+ * runtime SURVIVES.
  *
  * Production is unaffected: nothing fires unless kicadTestArmTimerPark is
  * called.