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

README.md

@@ -58,7 +58,7 @@ kicad-wasm/
 ├── tests/                  # Playwright E2E tests
 │   ├── e2e/                # Test specs
 │   └── apps/               # WASM test applications
-├── tools/                  # External tools (binaryen)
+├── tools/                  # External tools (local emsdk install)
 └── output/                 # Build output (pcbnew.js, pcbnew.wasm)
 ```
 
@@ -95,8 +95,8 @@ See [docs/build.md](docs/build.md) for detailed build documentation.
 #### Creating an isolated worktree (with submodule branches)
 
 For an experiment or feature you can work in a disposable git worktree so the
-main checkout stays pristine. This repo has four submodules
-(`kicad`, `wxwidgets`, `binaryen`, `web/pcbjam-shared`); a new worktree starts
+main checkout stays pristine. This repo has three submodules
+(`kicad`, `wxwidgets`, `web/pcbjam-shared`); a new worktree starts
 with them empty, so initialize and branch each one:
 
 ```bash
@@ -106,11 +106,11 @@ git worktree add -b experiment/my-thing ../kicad-wasm-my-thing main
 # 2. Check out the submodules INSIDE the worktree (working trees only;
 #    git objects are shared with the main checkout)
 cd ../kicad-wasm-my-thing
-git submodule update --init kicad wxwidgets binaryen web/pcbjam-shared
+git submodule update --init kicad wxwidgets web/pcbjam-shared
 
 # 3. Create a matching branch in each submodule (they start at detached HEAD)
 git checkout -b experiment/my-thing                 # root already on it via -b above
-for sm in kicad wxwidgets binaryen web/pcbjam-shared; do
+for sm in kicad wxwidgets web/pcbjam-shared; do
   git -C "$sm" checkout -b experiment/my-thing
 done
 ```
@@ -119,12 +119,11 @@ Then build from inside the worktree. Use an **isolated** Docker project — do N
 set `COMPOSE_PROJECT_NAME` to another branch's project (e.g. `kicad-wasm-main`),
 which can collide with other workflows; `docker/build.sh` auto-derives an isolated
 project name from the worktree branch. The first build provisions deps
-(wxWidgets + OCC) from scratch. To keep the machine responsive / bound wasm-opt
-RAM, cap parallelism and skip the slow release optimization:
+(wxWidgets + OCC) from scratch. To keep the machine responsive, cap
+parallelism:
 
 ```bash
-KICAD_DOCKER_CPUS=4 BINARYEN_CORES=4 BINARYEN_OPT_LEVEL=-O1 \
-  ./docker/build.sh pcbnew -j 4
+KICAD_DOCKER_CPUS=4 ./docker/build.sh pcbnew -j 4
 ```
 
 Tear down afterward with `git worktree remove ../kicad-wasm-my-thing` (and