[Blackwellcode]

Summary

Adds tests for the invariant functions that still had none (audit finding T1, 06-tests 1.3 a/c/d). No production code changes.

tests/test_invariants.py
├── ChannelLockFileTests              lock file carries the child PID; release only by holder / matching PID
├── InstanceLockRealFlockTests        (POSIX) second recorder instance exits 1 while the flock is held
├── StopChannelProcessRealChildTests  (POSIX, fork) worker ignoring SIGTERM is SIGKILLed within the deadline
└── WorkerVodTaskPathTests            _safe_vod_task_path rejects traversal, absolute, temp and symlinked names

Split out of #14, which keeps only the error-handling changes.

Evidence

• Before: worker._safe_vod_task_path 1/17 lines executed; try_acquire_channel_lock 1/23, update_channel_lock_pid 1/15, release_channel_lock 1/27 (local Windows coverage run).
  After: 17/17, 14/23, 10/15, 18/27. python -m unittest tests.test_invariants -> Ran 38 tests ... OK (skipped=11) on Windows.
• The instance-lock, SIGKILL and symlink tests only run on Linux (CI). In the CI run of fix: remove the last broad exception handlers, cover remaining invariants #14 they reported ok.

Merge Danger

Door: two-way

Blast Radius: none

Tests only. On Linux CI, the fork-based test adds about 0.5 s.

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