fix(firefox): require live terminal stdin

[kbuffardi]

Summary

• remove the pre-supplied/buffered stdin dialog, contract, WASI path, tests, and documentation
• require Firefox worker-side JSPI for live canonical terminal stdin
• report unsupported stdin without opening a dialog or posting a run when live input is unavailable
• preserve Chromium SharedArrayBuffer/Atomics input and Firefox JSPI prompt/EOF/Ctrl+C behavior

Closes #57
Related: #55, #56

Verification

• npm run test:e2e (169 tests)
• npm run lint
• npm run build
• npm run test:browser:firefox (0 errors; existing web-ext warnings)

Real Firefox JSPI prompt-by-prompt runtime acceptance remains a manual check documented in docs/firefox-stdin-runtime-acceptance.md.