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/kicad/footprint-chooser-close.spec.ts

@@ -104,6 +104,8 @@ test.describe('Add Footprint chooser close (doc-19 dead-app repro)', () => {
                 framesBefore,
                 { timeout: 40000 },
             )
+            // best-effort wait: the frame-count expect right below is the
+            // real assertion; a timeout must reach it, not throw (documented)
             .catch(() => {});
         const framesOpen = await frameCount(page);
         console.log(`[TEST] frames: ${framesBefore} → ${framesOpen}`);
@@ -131,6 +133,8 @@ test.describe('Add Footprint chooser close (doc-19 dead-app repro)', () => {
                 framesBefore,
                 { timeout: 20000 },
             )
+            // best-effort wait: the trap check below is the real assertion;
+            // a stalled close must reach it, not throw here (documented)
             .catch(() => {});
         await assertResponsive(page, 'chooser cancel');