feat(engine): implement protocol handshake negotiation

[Dencyuman]

Summary

• implement Engine Protocol 1.0 handshake negotiation with explicit protocol-range and schema-digest authority
• publish exact endpoint capabilities, transports, compiler limits, and stable success/rejected/failed/cancelled envelopes
• bound hostile handshake inputs and add deterministic Go, generated TypeScript, fixture, fuzz, and conformance coverage

Linked Issues

• Closes [Feature]: Implement Engine Protocol handshake and capability negotiation #28

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Scope

• Documentation only
• LDL / schema / protocol
• Engine / Runtime / Query / View
• SDK / MCP / client integration
• Server / storage / realtime / access
• Registry / packs / templates
• Release / governance / legal

Checks

• I did not add secrets, customer data, private deployment config, or local agent config.
• I checked the package boundary and licensing docs for affected paths.
• I updated generated artifacts or fixtures if a schema changed.
• I documented any public API, protocol, license, or release impact.
• I identified compatibility or migration impact, or confirmed there is none.
• I identified whether a release note is required.

Contract Impact

• Affected components and delivery bundles: Go Engine endpoint and generated protocol package
• LDL / schema / protocol / container impact: adds bounded Engine Protocol 1.0 handshake authority
• State or data migration impact: none
• License or dependency impact: none
• Release note: included in the portable compiler release set

Validation

• COREPACK_ENABLE_NETWORK=0 NODE_VERSION=24.18.0 /Users/dencyu/.nvm/nvm-exec make ci
• focused handshake, fuzz, race, generated-code, and cross-language fixture coverage