feat: add Firefox support via cross-browser build (Phase 8.2)

[Olawoyin007]

Summary

Phase 8.2: ports the extension to Firefox MV3 without a runtime browser sniff or a vendored polyfill. The same source runs on Chromium and Firefox; a small build step emits the per-browser manifests.

Changes

• extension/build.js (npm run build) - emits two bundles from the single source manifest.json:
  • dist/chrome/ - MV3 background.service_worker (unchanged)
  • dist/firefox/ - event-page background.scripts + the mandatory browser_specific_settings.gecko.id
• Cross-browser API shim in the three files that call extension APIs (background.js, core/classifier.js, popup/popup.js): a one-line globalThis.browser = globalThis.browser || globalThis.chrome alias, then chrome.* → browser.*. On Chromium browser aliases MV3's promise-based chrome; on Firefox browser.* is natively promise-based. Both keep the existing onMessage return true / sendResponse pattern, so no listener refactor.
• Docs: ROADMAP 8.2 code items checked off; CHANGELOG entry. dist/ gitignored.

Why the alias over webextension-polyfill

The roadmap floated the polyfill, but it is 40KB vendored and routes onMessage through a wrapper that expects promise-returning listeners - which would force a background.js refactor. The one-line alias is lighter, adds no dependency, and preserves the working return true/sendResponse pattern (native on both engines). Fits the project's minimal-surface ethos.

Firefox CSP

Verified there are no inline scripts, inline event handlers, or eval; popup.html loads popup.js externally, so the default MV3 CSP needs no content_security_policy override.

Testing

• cd extension && npm test - 81/81 Jest tests pass, unchanged (the alias resolves to the mocked chrome in jsdom)
• npm run build produces complete dist/chrome and dist/firefox bundles with correct manifests
• Manual smoke on Firefox Developer Edition (load dist/firefox via about:debugging, check Twitter/X + YouTube + Reddit classify and render) - human task
• No-regression check on one Chromium browser - human task

Notes

• content.js is unreferenced legacy (not in the manifest); left untouched and excluded from the build - flagging rather than removing.
• The Firefox add-on id (intentkeeper@olawoyin007.github.io) is a stable placeholder; change before AMO submission if you prefer a different namespace.