GRE-222: Recover browser media and control independently

[HeadCase]

Summary

• recover WHEP media and control WebSocket transports independently
• retry immediately, then with bounded jittered backoff that resets after stable recovery
• preserve the last trustworthy frame while media reconnects without replaying camera intent

Changes

• add generation-fenced reconnect loops with cancellation and online/foreground bypass
• bound aggregate WHEP negotiation and make remote session cleanup best-effort and non-blocking
• add retained-frame presentation and explicit reconnecting UI states
• expand unit, HTTP-contract, and Playwright recovery coverage
• record deployed acceptance evidence in docs/verification/gre-222-browser-recovery.md

Verification

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-targets --all-features
• cargo deny check
• cargo machete
• npm test
• npm run typecheck:browser
• deployed Mac Playwright acceptance against the real Rust, FFmpeg, MediaMTX, and ASI camera stack
• Standards review: clean
• Spec review: clean

Closes GRE-222