Harden screenshot redaction architecture

[neonwatty]

Closes #152

Summary

• Create immutable redaction snapshots at capture start and carry the same redaction metadata through the capture result used by annotation.
• Detect unsupported marked surfaces, including wrapper-marked canvas/image/video/iframe descendants, and warn during manual review without rescanning live DOM.
• Add edge fixture and E2E coverage for open Shadow DOM, closed-shadow host masking, selected-area warning scoping, auto-mode masking, and DOM mutation during capture.
• Clarify screenshot masking guarantees and limitations in developer docs.

Verification

• npm run validate
• npx playwright test e2e/widget.spec.ts --project=chromium -g "open shadow|closed shadow|marked unsupported surfaces|marked wrapper|selected-area capture warns|selected-area capture does not warn|auto mode applies|capture-start redaction geometry"
• CI=true make test-e2e-shard SHARD=1/2
• CI=true make test-e2e-shard SHARD=2/2

[github-actions]

🎉 This PR is included in version 1.37.4 🎉

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