jspi: migration phases 0-7 — build knob, scheduler shim, test successor suite

Toolchain: emsdk 6.0.6 (versions.sh; cache-hash keys on it). Build knob
PCBJAM_ASYNC_BACKEND=jspi|asyncify: build-kicad-target.sh links editors with
-sJSPI + -sJSPI_EXPORTS=@scripts/common/jspi-exports.txt + --pre-js
jspi-scheduler.js (no DYNCALLS, no post-link asyncify pipeline); wx build
stamps the backend and forces clean on flip or unknown provenance;
docker/build.sh passes the knob, seeds the emscripten ports cache from the
volume every launch, jspi postprocess = patch-env-shim only.

scripts/common/shims/jspi-scheduler.js: the JSPI successor scheduler —
token-wait registry, resume turnstile (one armed resume between engine
re-entries, SP swaps only at microtask boundaries), green-region spill
stacks (16-aligned tops), S1 embind mutator FIFO lane + parker wraps, S6
shutdown, libctx integration hooks (suspend/end/quarantine + g_current
arm/clear), SuspendError attributor, lost-wake + stuck-window watchdogs,
__wxWaitDump observability.

Embind: PARKER registrations get emscripten::async() under PCBJAM_JSPI
(wasm/bindings/pcbjam_async_policy.h). nanosleep yields route via the shim.

Tests: tests/asyncify -> tests/jspi successor suite (jspi-stack red/green
shadow-stack battery, jspi-coroutine MiniCoro harness, suspend-races
semantic scenarios + __wxWaitDump books coherence); projects jspi-firefox/
jspi-chrome (asyncify-webkit retired — no JSPI in WebKit); unconditional
Firefox JSPI pref; guard-beacons -> wait-beacons (+wxScheduler/libctxJspi
families); Makefile.wasm links test apps against JSPI with the shim as a
tracked link prerequisite.

Web: WasmTool setRo await + __wxWaitDump forensics, open-flow contained
promise, scheduler-shim.test.ts retargeted (8 green).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NDeBaKKhQztd8KiVtHuyXr

tests/apps/standalone/jspi-stack/build.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+# Ad-hoc build for the jspi-stack harness (Makefile.wasm wiring lands in Phase 3).
+# Uses the worktree-local emsdk (tools/emsdk, 6.0.6). EH flags mirror production.
+set -eo pipefail
+cd "$(dirname "$0")"
+ROOT="$(cd ../../../.. && pwd)"
+EMCC="${EMCC:-$ROOT/tools/emsdk/upstream/emscripten/em++}"
+
+COMMON=(
+  -O1
+  -fwasm-exceptions -sSUPPORT_LONGJMP=wasm -sWASM_LEGACY_EXCEPTIONS=1
+  -sJSPI -sJSPI_EXPORTS=activation
+  -sEXPORTED_RUNTIME_METHODS=stackSave,stackRestore,HEAPU8
+  -sMODULARIZE=1 -sEXPORT_ES6=1 -sENVIRONMENT=node,web,worker
+  -sALLOW_MEMORY_GROWTH=1
+)
+
+"$EMCC" stack_test.cpp -o stack_test.mjs "${COMMON[@]}" \
+  -sEXPORTED_FUNCTIONS=_activation,_stomp,_stack_current,_stack_base,_stack_end,_main,_malloc,_free
+
+"$EMCC" stack_test.cpp -o stack_test_pt.mjs "${COMMON[@]}" \
+  -pthread -sPTHREAD_POOL_SIZE=2 -sPTHREAD_POOL_SIZE_STRICT=0 \
+  -sEXPORTED_FUNCTIONS=_activation,_stomp,_stack_current,_stack_base,_stack_end,_main,_malloc,_free,_start_churn,_stop_churn
+
+echo "built: stack_test.mjs + stack_test_pt.mjs"