Harden browser handoff contracts

[chubes4]

Summary

• Add stable fanout identifiers, lifecycle progress counts, and structured skipped dependency results.
• Validate host-delegation request/result envelopes and return structured failures for malformed or stale inputs.
• Add browser-contained-site snapshot/export/apply-plan/apply-result contract DTOs and ability descriptors.

Verification

• npm run build
• npm run test:fanout-execution
• npm run test:agent-fanout-executor
• npm run test:fanout-aggregation-contract-parity
• npx tsx tests/host-delegation-validation.test.ts
• npm run test:browser-contained-site-status
• npm run test:runtime-contract-manifest
• npm run test:php-browser-contained-site-contract
• npm run test:public-ability-aliases
• npm run test:php-public-api-facade
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Implementing contract hardening, resolving clean-branch conflicts, adding tests/docs, and drafting this PR description.