Serialize config.toml writes with a cross-process lock (+ concurrency tests)

[alexkroman]

Concurrency tests

Adds tests/test_concurrency.py for the two genuinely contended runtime paths that lacked direct coverage:

• core.config — config.toml's read-modify-write under thread contention: a 24-writer + reader stress test pins that _dump's temp-file + atomic os.replace never lets a concurrent reader observe a truncated file.
• streaming.StreamSession.on_turn — runs on the SDK reader thread, and --system-audio drives two of them. A deterministic test proves the render + save + meta critical section holds _callback_lock (a second thread can't acquire it mid-save), plus a two-source stress test that every finalized turn lands in the saved transcript exactly once and intact.

Config write lock (the fix the tests motivated)

config.toml's mutating helpers do a read-modify-write (_load → mutate → _dump). The atomic os.replace already keeps a reader from seeing a torn file, but two assembly processes writing concurrently would still lose an update (both read the same config; the second dump clobbers the first). This wraps every read-modify-write in a cross-process filelock so concurrent writers no longer clobber each other; readers stay lock-free.

• filelock graduates from a dev-only transitive dependency to a declared runtime dependency.
• core/locking.py — generic cached cross-process FileLock helper (one instance per path, so nested acquisitions stay reentrant).
• core/config_lock.py — config.toml's lock_path/write_lock/locked/update helpers, kept out of config.py to stay under the file-length gate (_load/_dump injected so it avoids config's private API).
• set_* / clear_session / get_device_id / persist_login now route through the lock.

The lost-update concurrency test is correspondingly a no-loss test (16 concurrent writers each adding a distinct profile all survive), with unit tests pinning the lock path, per-dir rebuild, and that the lock is held across the dump.

Full scripts/check.sh is green (100% patch coverage, mutation gate, deptry, CodeQL). Rebased/merged onto latest main (which extracted keyring_store); the lock and that refactor compose cleanly.

🤖 Generated with Claude Code

https://claude.ai/code/session_01FueRkrQHWSfpPf1KNaZskX