feat(warmup): log canaries to stderr by default (with main-thread violation warning)

Per module: prints a one-line summary to stderr when the import
completes or fails:
  [warmup 1] google.genai on controller-io_0 (id=18636): 1218.6ms
  [warmup 2] anthropic on controller-io_1 (id=5500): 1148.3ms
  [warmup 3] openai on controller-io_2 (id=34376): 1144.2ms
  ...

When the entire warmup completes, prints an aggregate:
  [warmup done] 9 modules: 9 completed (sum of per-module elapsed: 3591.7ms)

If ANY canary ran on the main thread (main-thread-purity violation),
the per-module line is tagged with [MAIN-THREAD] AND a final WARNING
is printed:
  [warmup WARNING] N module(s) loaded on the MAIN THREAD: google.genai

Default is log_to_stderr=True so production runs get the observability
for free. Tests opt out via WarmupManager(pool, log_to_stderr=False)
in the _build_warmup helper.

5 new tests (4 stderr logging + 1 quiet). All 13 canary tests pass.

Use case: 'did my heavy import run on the GUI thread when it shouldnt
have?' is now answered by grepping stderr for [warmup ...] [MAIN-THREAD]
lines. No hook server required.

tests/test_warmup_canaries.py

@@ -27,9 +27,9 @@ from src.io_pool import make_io_pool
 
 
 def _build_warmup() -> tuple[WarmupManager, object]:
- """Build a fresh WarmupManager + pool for testing."""
+ """Build a fresh WarmupManager + pool for testing (silent by default)."""
  pool = make_io_pool()
- mgr = WarmupManager(pool)
+ mgr = WarmupManager(pool, log_to_stderr=False)
  return mgr, pool
 
 
@@ -164,3 +164,78 @@ def test_canary_canary_id_increments_across_resets() -> None:
   f"canary_ids should be [first=1, second=2]; got {second_ids}"
  )
  pool.shutdown(wait=True)
+
+def test_warmup_logs_to_stderr_on_completion(capsys: pytest.CaptureFixture) -> None:
+ """Successful canaries print a one-line summary to stderr."""
+ pool = make_io_pool()
+ mgr = WarmupManager(pool, log_to_stderr=True)
+ mgr.submit(["os", "json"])
+ assert mgr.wait(timeout=10.0)
+ captured = capsys.readouterr()
+ # Each completed module should have a log line
+ assert "[warmup" in captured.err
+ assert " os " in captured.err
+ assert " json " in captured.err
+ # Format: "[warmup N] module on thread (id=IDENT): ELAPSEDms"
+ assert "controller-io" in captured.err
+ assert "ms" in captured.err
+ pool.shutdown(wait=True)
+
+
+def test_warmup_can_be_quiet(capsys: pytest.CaptureFixture) -> None:
+ """log_to_stderr=False suppresses the per-module log lines."""
+ pool = make_io_pool()
+ mgr = WarmupManager(pool, log_to_stderr=False)
+ mgr.submit(["os", "json"])
+ assert mgr.wait(timeout=10.0)
+ captured = capsys.readouterr()
+ # No per-module log lines
+ assert "[warmup]" not in captured.err
+ # But the structured canary records still exist
+ canaries = mgr.canaries()
+ assert len(canaries) == 2
+ assert all(c["status"] == "completed" for c in canaries)
+ pool.shutdown(wait=True)
+
+
+def test_warmup_logs_total_time_at_completion(capsys: pytest.CaptureFixture) -> None:
+ """A summary line is printed when the entire warmup completes."""
+ pool = make_io_pool()
+ mgr = WarmupManager(pool, log_to_stderr=True)
+ mgr.submit(["os", "json"])
+ assert mgr.wait(timeout=10.0)
+ captured = capsys.readouterr()
+ # Summary line contains "done" or "ready" or "complete"
+ err_lines = [l for l in captured.err.splitlines() if l.strip()]
+ assert len(err_lines) >= 3  # 2 per-module + 1 summary
+ # The summary should mention total/total_ms or something aggregate
+ summary_line = err_lines[-1]
+ assert "warmup" in summary_line.lower()
+ pool.shutdown(wait=True)
+
+
+def test_warmup_logs_failure_to_stderr(capsys: pytest.CaptureFixture) -> None:
+ """A failed import prints a FAILED log line to stderr."""
+ pool = make_io_pool()
+ mgr = WarmupManager(pool, log_to_stderr=True)
+ mgr.submit(["definitely_does_not_exist_xyz_12345"])
+ assert mgr.wait(timeout=10.0)
+ captured = capsys.readouterr()
+ # Should contain a FAILED marker
+ assert "FAILED" in captured.err
+ assert "definitely_does_not_exist_xyz_12345" in captured.err
+ pool.shutdown(wait=True)
+
+
+def test_warmup_log_line_includes_thread_id(capsys: pytest.CaptureFixture) -> None:
+ """The log line includes the thread_id (matching the canary record)."""
+ pool = make_io_pool()
+ mgr = WarmupManager(pool, log_to_stderr=True)
+ mgr.submit(["os"])
+ assert mgr.wait(timeout=10.0)
+ canaries = mgr.canaries()
+ captured = capsys.readouterr()
+ # The thread_id from the canary should appear in the log
+ thread_id = str(canaries[0]["thread_id"])
+ assert thread_id in captured.err, f"expected thread_id {thread_id} in stderr: {captured.err!r}"
+ pool.shutdown(wait=True)