fix(linux): guard host state against concurrent shutdown

[yashranaway]

Closes #12 — the one priority:critical item in Phase 1.

The race

shutdown deliberately bypasses the transport's request queue so an operator can always stop a stalled browser action (Transport.swift:213-218). That is a feature, and this PR keeps it. The consequence is that host.stop() runs on the main thread while a normal command is still executing on a client thread.

LinuxBrowserHost held sessions, trace, activeFlows, and recordings as plain dictionaries behind @unchecked Sendable with no lock. Two threads mutating the same Swift dictionary is not a lost update — it is memory corruption. stop() clearing recordings while a record start inserted into it can crash the host process and take every session with it.

The macOS host already serialises the same state through onAgentMain plus a dedicated recordings lock. Linux had nothing. Asymmetric safety for identical state.

The fix

• All four containers go through a stateLock. The lock is held only around collection access, never across browser I/O, so a stalled command still cannot delay teardown — the exact property the queue bypass exists to provide.
• stop() is idempotent: flips a stopping flag, snapshots and clears the containers under the lock, then stops recordings, closes sessions, and kills the browser outside it.
• Session and recording registration re-check stopping under the lock and back out cleanly if teardown has begun. Without that, a recording started during the teardown window leaks an FFmpeg process nothing will ever stop, and a session created in the same window leaks a browser target.

Test

Tests/linux-e2e.sh now drives the real race rather than a proxy: an active recording and a full-page tour in flight, then headless stop, asserting the host exits on its own within 30 s and restarts clean with only the default session. It runs in the Linux E2E (Docker) job.

Verification note

I cannot compile Swift in my environment — no toolchain, and no Docker access — so CI is the first compile of this change. I re-read every edited region and specifically checked for the Swift trap where a helper named session/recording would resolve to a local variable declared later in the same scope; the helpers are named lookupSession/lookupRecording to avoid it. If CI finds something, I will fix it here rather than in a follow-up.