feat(firefox): add JSPI live terminal stdin

[kbuffardi]

Summary

• add Firefox 153+ live, line-buffered terminal stdin through WebAssembly JSPI
• negotiate JSPI from the compiler worker and route session-scoped stdin data and EOF messages
• preserve the existing Chromium SharedArrayBuffer/Atomics path as the first transport
• retain the pre-supplied buffered fallback for Firefox 140-152 and runtimes without JSPI
• document manual Firefox runtime acceptance; persistent folder write-back remains out of scope

Verification

Passed:

• npm ci
• npm run fetch-clang
• npm run lint
• npm run build
• npm run test:e2e (177 tests)
• npm run test:preflight-clang
• npm run test:browser:firefox (0 errors; 15 existing web-ext warnings)
• npm run version:check
• npm run release:check-version

Inconclusive/manual:

• npm run test:browser:chrome passed preflight but the browser smoke produced no completion or diagnostic after an extended wait and was terminated
• real Firefox 153+ extension execution is not covered by the packaging smoke; follow docs/firefox-stdin-runtime-acceptance.md before release

Scope

Persistent Firefox folder write-back and directory-handle restoration are intentionally unchanged.

Closes #55

Requested by @kbuffardi: "implement this plan and raise a pr"

Implemented by Codex (GPT-5).