design: Browser Runtime Control Plane

[jackwener]

Summary

Complete design document for upgrading the daemon/extension layer from a "background process on a fixed port" to a structured Browser Runtime Control Plane.

Covers:

• Runtime Identity: generationId, extension session identity
• Hello v2 Protocol: capability handshake with backward compatibility
• Capability Routing Table: native/fallback/unavailable paths for every browser capability
• Structured Health Model: 4-dimensional health (transport/compatibility/capability/commandLane)
• Transactional Lifecycle: confirmed stop, draining, replace state machine
• Correctness Fixes: command ID collision, timeout mismatch, routing bug, cleanup inconsistency
• Scenario Matrix: 15 cross-version scenarios with expected behavior at every layer
• Review & Merge Gate: 3-layer review, regression suite, release checklist

This is a design-only PR (markdown). No code changes. Requesting review from @codex-coder and @First-principles-0 before implementation begins.

Review Focus

1. Is the hello v2 schema complete and backward-compatible?
2. Is the capability routing table accurate for all current features?
3. Does the scenario matrix cover all real-world cross-version combinations?
4. Is the lifecycle state machine correct and complete?
5. Are the correctness fixes sufficient?

Test plan

• Design review by implementation reviewer
• Design review by first-principles reviewer
• Scenario matrix validated against current codebase
• No code changes — doc-only PR