test(compat): add a Firefox smoke lane for source selection

[stackloomdev]

Context

Current browser E2E coverage runs in Chromium. The stable-release milestone needs one independent Firefox smoke path before broader browser claims are made.

Scope

• Add a focused Firefox Playwright project or CI lane for the React 19 fixture.
• Select ordinary static text and verify the exact TSX location.
• Trigger one real state update, select the affected UI, and verify its transition evidence.
• Export the selected trace and confirm the result is redacted and structurally valid.
• Document the exact Firefox version tested; do not broaden the compatibility table beyond the automated evidence.

Acceptance

• Firefox coverage runs from a clean install in CI.
• The lane is bounded and does not duplicate the full Chromium suite.
• pnpm check and the new lane pass.