perf(desktop): coalesce control admission and observe viewer transmit stalls

[rldyourmnd]

Short control frames previously admitted the length prefix and body in two writes. Serialize them into one buffer before write_all, preserving legacy wire bytes, partial-write handling and the size bound. This removes a separate prefix-only admission; it does not guarantee one packet or cancellation-safe writes. Ten wire tests pass.

Slow desktop input/control replies also need viewing-endpoint transmit evidence: server TX counters cannot identify viewer TX loss. Add a weak backend-neutral path observer and a separate bounded observation worker for matched control replies. Numeric process-local control-instance IDs join writes, reads and path observations.

The worker owns one synchronous query and one newest queued request, never waits on input/receipt admission or event reads, and cannot terminate the desktop session if diagnostics fail. Snapshots retain eight live baselines, prefer selected paths, expose coverage/truncation/age/evictions, and preserve unknown/reset deltas. No wire, timeout, endpoint settings, identity or authority changes; no addresses, input/clipboard bodies or credentials.

Validation: macOS strict full-workspace/all-targets desktop/Noq Clippy passed; 307 targeted desktop/transport tests passed, including actual Iroh and owned-Noq observation/connection lifecycle tests. Initial linking hit ENOSPC; the guarded retry passed without a disk guard trigger. Formatting and private-data boundary checks passed. The new framing change requires renewed full workspace/cross-platform checks; those are running. Linux/installed traces remain required. This repairs a diagnostic gap; residual native pauses are still unaccepted.