Exercise the production browser lifecycle

[jt55401]

What changed

• replace copied content-script test helpers with direct tests of production lifecycle functions
• prove source-snapshot mapping fails closed and navigation reset uses the new snapshot
• prove nested markers remain outside the outermost signed section
• prove stale/source-only states render warnings and mutations trigger re-verification
• refresh browser-client and canonicalization dependency pins

Verification

• npm test -- --runInBand (63 passed)
• npm run typecheck
• npm run build:all
• npm run lint (0 errors; existing unrelated warnings remain)