Classify Wasm traps as POSIX signals

[brandonpayton]

Purpose

Kandelo was reporting unexpected Wasm process traps through the POSIX-shaped wait status as generic SIGSEGV/139, so users saw Segmentation fault even when the engine trap was arithmetic or illegal control flow. This change makes those crash statuses more informative while preserving the existing wait-status contract.

The goal is not to claim perfect POSIX provenance from Wasm. It is to use the trap reason exposed by the engine when it is reliable enough, keep SIGSEGV for memory/stack/generic bounds failures, and avoid masking arbitrary user-space unreachable traps as successful exits.

Summary

• add shared Wasm trap-to-signal classification for memory/bounds, stack, arithmetic, and illegal control-flow traps
• propagate classified signal numbers through Node and browser process-worker crash finalization
• reject non-Wasm executable bytes before worker launch with ENOEXEC, so native host binaries cannot surface as opaque WebAssembly.compile() crashes
• keep guest examples from inheriting the host PATH, preventing posix_spawnp() from resolving native host tools such as /usr/bin/echo
• make local Node workers avoid stale ignored host/dist/node-kernel-worker-entry.js artifacts when the TypeScript source is newer
• cover trap classification in host tests and deterministic Chromium/Firefox/WebKit browser tests
• make host kernel worker changes trigger browser smoke coverage, including the trap classifier spec

Notes

• Firefox reports both linear-memory and table bounds traps as RuntimeError: index out of bounds, so the classifier treats generic Wasm bounds traps as SIGSEGV rather than claiming a memory/table distinction.
• The staging browser suite runs the full fast app suite in Chromium and the trap classifier spec in Chromium/Firefox. WebKit trap-classifier coverage runs in the Browser demo smoke workflow because WebKit is not reliable inside the Nix staging browser job.
• Replaces fork PR Classify Wasm traps as POSIX signals #680 so the same-repo staging/test gate can run.

Validation

• nix develop -c bash -c 'cd host && npm exec vitest -- run test/trap-signals.test.ts test/wasm-trap.test.ts test/centralized-spawn.test.ts'
• nix develop -c bash -c 'scripts/run-libc-tests.sh functional spawn'
• nix develop -c bash -c 'cd apps/browser-demos && CI=true KANDELO_PLAYWRIGHT_PORT=5414 PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD=1 npx playwright test wasm-trap-signal.spec.ts --project=chromium --project=firefox --project=webkit --workers=1'
• nix develop -c bash -c 'cd host && npm run build'
• Browser demo smoke tests: https://github.com/Automattic/kandelo/actions/runs/27442470901
• Staging build: https://github.com/Automattic/kandelo/actions/runs/27442470918

[github-actions]

Phase B-1 matrix build status — pr-685-staging

ABI v15. 5 built, 0 failed, 5 total.

Package	Arch	Status	Sha
kandelo-sdk	wasm32	built	130a0f57
lamp	wasm32	built	16a623c3
shell	wasm32	built	d8126d7e
wordpress	wasm32	built	803c15e7
node-vfs	wasm32	built	3963ff62

_{Auto-generated; replaced on each push. Raw data in the publish-status workflow artifact.}

[github-actions]

prepare-merge: test-gate passed against the synthetic PR merge and binaries-abi-v15. Missing entries were built or promoted from PR staging only when their merged-tree cache key matched; merge-gate=success posted on PR HEAD and squash auto-merge enabled.