feat(spectrum): capture macOS output with an avfoundation loopback

SYNC mode was Linux-only: it shelled out to parec. On macOS there is no
PulseAudio, so the analyser always fell back to the drifting stream tap.

Capture with 'ffmpeg -f avfoundation' from a loopback input instead
(BlackHole, Loopback, Soundflower, Multi-Output/Aggregate devices are
auto-detected; SOMAFM_SPECTRUM_DEVICE accepts a name or index). Plain
microphones are never auto-selected.

A loopback that nothing is routed into delivers endless digital silence,
so treat ~6 s of all-zero blocks as a dead monitor and fall back to the
stream tap rather than showing frozen bars. parec handling is unchanged.

tests/test_spectrum.py

@@ -138,3 +138,113 @@ def test_stream_tap_pipeline(tmp_path) -> None:
         assert engine.snapshot().status == "idle"
     finally:
         engine.stop()
+
+
+def test_avfoundation_loopback_is_preferred_on_macos(monkeypatch) -> None:
+    engine = SpectrumEngine(ffmpeg_path="/opt/homebrew/bin/ffmpeg")
+    engine._parec = None  # pretend there is no PulseAudio on this host
+    engine._darwin = True
+    monkeypatch.delenv("SOMAFM_SPECTRUM_DEVICE", raising=False)
+    monkeypatch.setattr(
+        engine,
+        "_avf_audio_devices",
+        lambda: [(0, "MacBook Pro Microphone"), (1, "BlackHole 2ch")],
+    )
+    engine._avf_checked = False
+
+    assert engine._avf_loopback() == 1
+    assert engine._monitor_kind() == "avfoundation"
+    assert engine.monitor_available is True
+
+    command = engine._monitor_command()
+    assert command is not None
+    assert command[0] == "/opt/homebrew/bin/ffmpeg"
+    assert command[command.index("-i") + 1] == ":1"
+    assert "-f" in command and "avfoundation" in command
+    assert command[-1] == "-"
+    assert "pcm_s16le" in command
+    assert command[command.index("-ar") + 1] == "44100"
+
+
+def test_avfoundation_never_autoselects_a_microphone(monkeypatch) -> None:
+    engine = SpectrumEngine(ffmpeg_path="/opt/homebrew/bin/ffmpeg")
+    engine._parec = None
+    engine._darwin = True
+    monkeypatch.delenv("SOMAFM_SPECTRUM_DEVICE", raising=False)
+    monkeypatch.setattr(
+        engine, "_avf_audio_devices", lambda: [(0, "MacBook Pro Microphone")]
+    )
+    engine._avf_checked = False
+    assert engine._avf_loopback() is None
+    assert engine._monitor_kind() is None
+    assert engine._monitor_command() is None
+
+
+def test_avfoundation_device_override_by_name_and_index(monkeypatch) -> None:
+    devices = [(0, "MacBook Pro Microphone"), (1, "BlackHole 2ch"), (2, "Loopback Audio")]
+
+    engine = SpectrumEngine(ffmpeg_path="/opt/homebrew/bin/ffmpeg")
+    engine._parec = None
+    engine._darwin = True
+    monkeypatch.setattr(engine, "_avf_audio_devices", lambda: devices)
+    monkeypatch.setenv("SOMAFM_SPECTRUM_DEVICE", "loopback audio")
+    engine._avf_checked = False
+    assert engine._avf_loopback() == 2
+
+    engine = SpectrumEngine(ffmpeg_path="/opt/homebrew/bin/ffmpeg")
+    engine._parec = None
+    engine._darwin = True
+    monkeypatch.setattr(engine, "_avf_audio_devices", lambda: devices)
+    monkeypatch.setenv("SOMAFM_SPECTRUM_DEVICE", "0")
+    engine._avf_checked = False
+    assert engine._avf_loopback() == 0
+
+
+def test_avfoundation_listing_parser(monkeypatch) -> None:
+    listing = (
+        "[AVFoundation indev @ 0x1] AVFoundation video devices:\n"
+        "[AVFoundation indev @ 0x1] [0] Capture screen 0\n"
+        "[AVFoundation indev @ 0x1] AVFoundation audio devices:\n"
+        "[AVFoundation indev @ 0x1] [0] MacBook Pro Microphone\n"
+        "[AVFoundation indev @ 0x1] [1] BlackHole 2ch\n"
+    )
+
+    class Result:
+        stdout = listing
+        returncode = 1
+
+    def fake_run(*args, **kwargs):
+        return Result()
+
+    engine = SpectrumEngine(ffmpeg_path="/opt/homebrew/bin/ffmpeg")
+    monkeypatch.setattr(spectrum_module.subprocess, "run", fake_run)
+    assert engine._avf_audio_devices() == [
+        (0, "MacBook Pro Microphone"),
+        (1, "BlackHole 2ch"),
+    ]
+
+
+def test_dead_loopback_falls_back_instead_of_flat_bars(tmp_path) -> None:
+    script = tmp_path / "silent-parec"
+    script.write_text(
+        "#!/usr/bin/env python3\n"
+        "import sys\n"
+        "block = b'\\0' * 4096\n"
+        "for _ in range(1000):\n"
+        "    sys.stdout.buffer.write(block)\n"
+        "sys.stdout.buffer.flush()\n"
+    )
+    script.chmod(0o755)
+
+    engine = SpectrumEngine(ffmpeg_path="")
+    engine.set_source("https://example.invalid/live")
+    proc = engine._spawn([str(script)], "monitor")
+    assert proc is not None
+    assert engine.snapshot().status == "monitor"
+    try:
+        assert engine._capture(
+            proc, "https://example.invalid/live", stop_on_url_change=False, status="monitor"
+        ) is False
+    finally:
+        engine._stop_proc(proc)
+        engine.stop()