fix: build PipeWire capture chain on demand instead of permanently

The null sink and its two loopbacks were loaded at startup and stayed
loaded forever. While loaded they link the microphone and the speaker
device into a single PipeWire driver group: the microphone drives the
graph and the speaker device runs as a clock follower, so every playback
stream pays for continuous cross-device resampling.

Nothing needs the chain while idle — recording is triggered manually.
Build it in toggle_recording() and drop it as soon as the audio is
captured, before transcription runs for minutes.

Module ids are now derived from pactl instead of tracked in a state file,
so a crashed process cannot leave stale ids behind. The state file only
holds the device selection. Startup tears down any leftover chain.

Loopbacks get latency_msec=200; transcription is offline, and a small
buffer would put these nodes on the graph's realtime deadline.

tests/test_pipewire.py

@@ -0,0 +1,145 @@
+import json
+from unittest.mock import patch, MagicMock
+
+import pytest
+
+
+@pytest.fixture
+def state_path(tmp_path, monkeypatch):
+    import pipewire
+    p = tmp_path / "pipewire-modules.json"
+    monkeypatch.setattr(pipewire, "STATE_PATH", p)
+    return p
+
+
+def test_save_devices_persists_selection(state_path):
+    import pipewire
+    pipewire.save_devices("mic-src", "monitor-src")
+    data = json.loads(state_path.read_text())
+    assert data["mic"] == "mic-src"
+    assert data["monitor"] == "monitor-src"
+
+
+def test_save_devices_does_not_load_modules(state_path):
+    import pipewire
+    with patch("subprocess.check_output") as run:
+        pipewire.save_devices("mic-src", "monitor-src")
+    run.assert_not_called()
+
+
+def test_load_devices_returns_none_without_state(state_path):
+    import pipewire
+    assert pipewire.load_devices() is None
+
+
+def test_load_devices_returns_none_on_incomplete_state(state_path):
+    import pipewire
+    state_path.write_text(json.dumps({"mic": "mic-src"}))
+    assert pipewire.load_devices() is None
+
+
+def test_load_devices_roundtrip(state_path):
+    import pipewire
+    pipewire.save_devices("mic-src", "monitor-src")
+    assert pipewire.load_devices() == {"mic": "mic-src", "monitor": "monitor-src"}
+
+
+def test_setup_without_configured_devices_does_nothing(state_path):
+    import pipewire
+    with patch("subprocess.check_output") as run:
+        assert pipewire.setup() == []
+    run.assert_not_called()
+
+
+def test_setup_loads_sink_and_two_loopbacks(state_path):
+    import pipewire
+    pipewire.save_devices("mic-src", "monitor-src")
+    with patch.object(pipewire, "is_loaded", side_effect=[False, True]), \
+         patch("subprocess.check_output", side_effect=[b"11\n", b"12\n", b"13\n"]) as run:
+        ids = pipewire.setup()
+    assert ids == [11, 12, 13]
+    cmds = [c.args[0] for c in run.call_args_list]
+    assert cmds[0][:3] == ["pactl", "load-module", "module-null-sink"]
+    assert cmds[1][:3] == ["pactl", "load-module", "module-loopback"]
+    assert cmds[2][:3] == ["pactl", "load-module", "module-loopback"]
+
+
+def test_setup_uses_configured_sources(state_path):
+    import pipewire
+    pipewire.save_devices("mic-src", "monitor-src")
+    with patch.object(pipewire, "is_loaded", side_effect=[False, True]), \
+         patch("subprocess.check_output", side_effect=[b"11\n", b"12\n", b"13\n"]) as run:
+        pipewire.setup()
+    cmds = [c.args[0] for c in run.call_args_list]
+    assert "source=mic-src" in cmds[1]
+    assert "source=monitor-src" in cmds[2]
+
+
+def test_setup_applies_large_loopback_latency(state_path):
+    """A small buffer would put the loopbacks on the graph's realtime deadline."""
+    import pipewire
+    pipewire.save_devices("mic-src", "monitor-src")
+    with patch.object(pipewire, "is_loaded", side_effect=[False, True]), \
+         patch("subprocess.check_output", side_effect=[b"11\n", b"12\n", b"13\n"]) as run:
+        pipewire.setup()
+    expected = f"latency_msec={pipewire.LOOPBACK_LATENCY_MSEC}"
+    for cmd in [c.args[0] for c in run.call_args_list][1:]:
+        assert expected in cmd
+    assert pipewire.LOOPBACK_LATENCY_MSEC >= 100
+
+
+def test_setup_is_noop_when_already_loaded(state_path):
+    import pipewire
+    pipewire.save_devices("mic-src", "monitor-src")
+    with patch.object(pipewire, "is_loaded", return_value=True), \
+         patch("subprocess.check_output") as run:
+        assert pipewire.setup() == []
+    run.assert_not_called()
+
+
+def test_teardown_unloads_chain_modules_highest_id_first():
+    import pipewire
+    listing = (
+        b"7\tmodule-something\tunrelated\n"
+        b"11\tmodule-null-sink\tsink_name=transkriptor-combined\n"
+        b"12\tmodule-loopback\tsource=mic-src sink=transkriptor-combined\n"
+        b"13\tmodule-loopback\tsource=monitor-src sink=transkriptor-combined\n"
+    )
+    with patch("subprocess.check_output", return_value=listing), \
+         patch("subprocess.run") as run:
+        pipewire.teardown()
+    unloaded = [c.args[0][-1] for c in run.call_args_list]
+    assert unloaded == ["13", "12", "11"]
+
+
+def test_teardown_ignores_unrelated_modules():
+    import pipewire
+    listing = b"7\tmodule-something\tunrelated\n"
+    with patch("subprocess.check_output", return_value=listing), \
+         patch("subprocess.run") as run:
+        pipewire.teardown()
+    run.assert_not_called()
+
+
+def test_teardown_survives_pactl_failure():
+    import pipewire
+    import subprocess as sp
+    with patch("subprocess.check_output", side_effect=sp.SubprocessError("boom")):
+        pipewire.teardown()  # must not raise
+
+
+def test_setup_survives_pactl_failure(state_path):
+    import pipewire
+    import subprocess as sp
+    pipewire.save_devices("mic-src", "monitor-src")
+    with patch.object(pipewire, "is_loaded", return_value=False), \
+         patch("subprocess.check_output", side_effect=sp.SubprocessError("boom")):
+        assert pipewire.setup() == []
+
+
+def test_is_loaded_detects_sink():
+    import pipewire
+    with patch("subprocess.check_output", return_value=b"42\ttranskriptor-combined.monitor\n"):
+        assert pipewire.is_loaded() is True
+    with patch("subprocess.check_output", return_value=b"42\tsomething-else.monitor\n"):
+        assert pipewire.is_loaded() is False