fix(watch): unlink .rebuild.lock on release and rewrite single PID line (#858)

The rebuild lock file accumulated concatenated PIDs across post-commit
rebuilds without a separator, and was never removed when the rebuild
finished. Two practical consequences for users:

1. Downstream tooling that polls for `.rebuild.lock` to disappear before
   doing post-rebuild work (publish scripts copying graph.html to a web
   root, etc.) blocked forever / until its own timeout.
2. The accumulated digit string could not be parsed by humans or tooling
   to find the owning PID.

The `_rebuild_lock` context manager now:

- Opens the lock file with `a+` so a non-acquiring caller does not
  truncate the existing holder's PID.
- After flock acquisition, truncates and writes a single `<pid>\n` line
  so external readers can `kill -0 $(cat .rebuild.lock)` to check
  liveness.
- Unlinks the lock file in the finally block (only when *we* held the
  lock), restoring the "signal-by-absence" convention users rely on.

Four regression tests added under `tests/test_watch.py` covering the
PID-with-newline payload, post-release unlink, no-accumulation across
sequential acquisitions, and the non-blocking-caller-does-not-clobber
invariant.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_watch.py

@@ -1,9 +1,11 @@
 """Tests for watch.py - file watcher helpers (no watchdog required)."""
+import os
+import sys
 import time
 from pathlib import Path
 import pytest
 
-from graphify.watch import _notify_only, _WATCHED_EXTENSIONS
+from graphify.watch import _notify_only, _WATCHED_EXTENSIONS, _rebuild_lock
 
 
 # --- _notify_only ---
@@ -96,6 +98,60 @@ def test_watch_raises_without_watchdog(tmp_path, monkeypatch):
         watch(tmp_path)
 
 
+# --- _rebuild_lock (GH-858) ---
+
+
+@pytest.mark.skipif(sys.platform == "win32", reason="fcntl-only (POSIX)")
+def test_rebuild_lock_writes_pid_with_newline(tmp_path):
+    out = tmp_path / "graphify-out"
+    lock_path = out / ".rebuild.lock"
+    with _rebuild_lock(out) as got:
+        assert got is True
+        assert lock_path.exists()
+        contents = lock_path.read_text(encoding="utf-8")
+        assert contents == f"{os.getpid()}\n", contents
+
+
+@pytest.mark.skipif(sys.platform == "win32", reason="fcntl-only (POSIX)")
+def test_rebuild_lock_removed_after_release(tmp_path):
+    """GH-858: lock file must be unlinked once the rebuild completes so
+    downstream waiters that poll for its absence unblock promptly."""
+    out = tmp_path / "graphify-out"
+    lock_path = out / ".rebuild.lock"
+    with _rebuild_lock(out) as got:
+        assert got is True
+    assert not lock_path.exists(), "lock file should be unlinked after release"
+
+
+@pytest.mark.skipif(sys.platform == "win32", reason="fcntl-only (POSIX)")
+def test_rebuild_lock_does_not_accumulate_pids_across_runs(tmp_path):
+    """GH-858: each acquisition truncates and rewrites the PID line rather
+    than appending, so the file never grows into a digit-concatenation."""
+    out = tmp_path / "graphify-out"
+    lock_path = out / ".rebuild.lock"
+    expected = f"{os.getpid()}\n"
+    for _ in range(5):
+        with _rebuild_lock(out) as got:
+            assert got is True
+            assert lock_path.read_text(encoding="utf-8") == expected
+        assert not lock_path.exists()
+
+
+@pytest.mark.skipif(sys.platform == "win32", reason="fcntl-only (POSIX)")
+def test_rebuild_lock_non_blocking_does_not_clobber_holder(tmp_path):
+    """GH-858: a non-blocking caller that fails to acquire the lock must not
+    truncate the holder's PID payload."""
+    out = tmp_path / "graphify-out"
+    lock_path = out / ".rebuild.lock"
+    with _rebuild_lock(out) as outer:
+        assert outer is True
+        held_contents = lock_path.read_text(encoding="utf-8")
+        with _rebuild_lock(out, blocking=False) as inner:
+            assert inner is False
+            # Holder's PID line must still be intact.
+            assert lock_path.read_text(encoding="utf-8") == held_contents
+
+
 def test_rebuild_code_is_idempotent_when_cluster_ids_flap(tmp_path, monkeypatch):
     from graphify import cluster as cluster_mod
     from graphify.watch import _rebuild_code