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

scripts/common/env.sh

@@ -41,8 +41,8 @@ export EMSDK_QUIET=1
 # WebAssembly exceptions (legacy binary encoding) are the only build mode. -sSUPPORT_LONGJMP=wasm is
 # required because the deps that use setjmp/longjmp (freetype, cairo, OpenCASCADE) must use wasm
 # setjmp — emscripten's JS-longjmp implementation cannot coexist with -fwasm-exceptions (it would
-# leave emscripten_longjmp undefined). -sWASM_LEGACY_EXCEPTIONS=1 selects the EH binary encoding our
-# post-link Asyncify + catch-arm-hoisting pass can consume (Asyncify can't handle exnref).
+# leave emscripten_longjmp undefined). -sWASM_LEGACY_EXCEPTIONS=1 pins the legacy EH binary encoding:
+# the exnref encoding is not adopted across our pinned emsdk toolchain and target browsers.
 export DEPS_EH_FLAGS="-fwasm-exceptions -sSUPPORT_LONGJMP=wasm -sWASM_LEGACY_EXCEPTIONS=1"
 
 # Emscripten SDK setup
@@ -123,23 +123,6 @@ elif [ -z "$JOBS" ]; then
     export JOBS=1
 fi
 
-# Common linker flags for WASM
-export WASM_LDFLAGS="\
--sALLOW_MEMORY_GROWTH=1 \
--sINITIAL_MEMORY=256MB \
--sSTACK_SIZE=5MB \
--sASYNCIFY=1 \
--sASYNCIFY_STACK_SIZE=16384 \
--sLEGACY_GL_EMULATION \
--sMAX_WEBGL_VERSION=2"
-
-# Threading flags (when enabled)
-export PTHREAD_LDFLAGS="\
--pthread \
--sPROXY_TO_PTHREAD=1 \
--sPTHREAD_POOL_SIZE=navigator.hardwareConcurrency \
--sOFFSCREENCANVAS_SUPPORT=1"
-
 # Create output directories
 mkdir -p "$BUILD_ROOT" "$DEPS_ROOT" "$SYSROOT"/{lib,include,share} "$STAMPS_DIR"