Fix atomic rewatch lock acquisition

[cknitt]

Summary

• Make rewatch lock acquisition atomic so concurrent processes cannot both acquire the same lockfile.
• Update watcher test helpers to wait for spawned watcher processes to exit after removing watch.lock.
• Adapt the lock/watch tests so sequential test runs do not leave overlapping watchers in the same fixture workspace.

Why

CI was intermittently failing with corrupted compiled interface artifacts while running rewatch tests. The root causes were lock/test races:

• lock acquisition used a check-then-create flow, allowing concurrent processes to both believe they acquired the same lock;
• the shell test helper removed watch.lock but did not wait for the watcher process to terminate before the next test started.

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commit: 1523f26