Harden browser lifecycle verification

[jt55401]

What changed

• fail closed when source snapshots are missing or cannot map completely to the live page
• keep indicators outside the outermost signed section and invalidate them after mutation
• reset source snapshots across navigation and reload
• exercise the production DOM walker in Chromium, including its serialized page callback
• refresh every coordinated implementation pin

Verification

• npm test (33 passed)
• npm run build
• npm run test:browser
• npm run check
• npm run e2e:small (all phases passed; 18/18 verification successes and source/rendered matches)