feat(canary): single-shot on_pause_callback + wire Telegram drift alert

Canary auto-pause was silent: when drift > threshold the module flipped
to paused without any user-facing notification, leaving the user to
wonder why detection went dark. Add an optional on_pause_callback
invoked exactly once per not_paused→paused transition. Wrap the call
in try/except so a notifier failure can never break the detection
cycle.

main.py wires the callback to emit canary_drift_paused audit event
plus a warn Alert guiding the user toward /resume or recalibration.

Tests: test_canary_pause_callback_fires_once (idempotent),
test_canary_resume_allows_new_pause_notification (re-arms after
resume), test_canary_pause_callback_exception_does_not_crash_check
(safety).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_canary.py

@@ -140,6 +140,52 @@ def test_pause_file_written(tmp_path: Path) -> None:
     assert flag.exists()
 
 
+def test_canary_pause_callback_fires_once() -> None:
+    """Single-shot: callback invoked exactly once per not_paused→paused edge."""
+    cfg = _cfg_with_baseline(BASELINE_FRAME)
+    calls: list[int] = []
+
+    canary = Canary(cfg, on_pause_callback=lambda d: calls.append(d))
+
+    canary.check(DRIFTED_FRAME)   # transition → callback fires
+    canary.check(DRIFTED_FRAME)   # still paused → no new callback
+    canary.check(BASELINE_FRAME)  # clean but still paused → no new callback
+
+    assert len(calls) == 1
+    assert calls[0] > 0  # distance should be positive
+
+
+def test_canary_resume_allows_new_pause_notification() -> None:
+    """After resume, a fresh drift must re-fire the callback."""
+    cfg = _cfg_with_baseline(BASELINE_FRAME)
+    calls: list[int] = []
+
+    canary = Canary(cfg, on_pause_callback=lambda d: calls.append(d))
+
+    canary.check(DRIFTED_FRAME)
+    assert len(calls) == 1
+
+    canary.resume()
+    canary.check(DRIFTED_FRAME)  # new pause transition
+
+    assert len(calls) == 2
+
+
+def test_canary_pause_callback_exception_does_not_crash_check() -> None:
+    """A failing callback must not break canary.check (detection cycle safety)."""
+    cfg = _cfg_with_baseline(BASELINE_FRAME)
+
+    def _boom(_d: int) -> None:
+        raise RuntimeError("notifier down")
+
+    canary = Canary(cfg, on_pause_callback=_boom)
+
+    # Must not raise — exception is swallowed + logged.
+    result = canary.check(DRIFTED_FRAME)
+    assert result.paused is True
+    assert canary.is_paused is True
+
+
 def test_resume_deletes_pause_file(tmp_path: Path) -> None:
     """resume() deletes the pause flag file."""
     flag = tmp_path / "paused.flag"