feat(spectrum): capture macOS output with a Core Audio process tap

SYNC mode on macOS needed a virtual loopback driver (BlackHole, Loopback,
Soundflower) routed through a Multi-Output Device, and shelled out to
'ffmpeg -f avfoundation' to read it. macOS 14.2 lets a process tap the
system mix directly, so do that instead: CATapDescription ->
AudioHardwareCreateProcessTap -> private aggregate device -> IOProc,
straight through ctypes. No driver, no helper binary, no ffmpeg.

The tap hands out native-rate interleaved float32, so the analyser can
now retune its window and band edges when the capture rate differs from
44.1 kHz, and mono-downmixes the frames before the FFT.

A tap that was never granted System Audio Recording permission is
created happily but only ever delivers digital silence: treat that as a
failed capture instead of flat bars, explain it in the status bar and
fall back to the stream tap. A tap that renders nothing *yet* (playback
still starting, or mpv pointed at another device) is not a permanent
failure either - the stream tap covers for a minute, then the tap is
retried.

Drop the avfoundation device enumeration, the loopback-name heuristics
and the microphone safeguards' tests, and document the new permission
and its diagnostics instead.

somafm_tui/__main__.py

@@ -55,7 +55,7 @@ def _build_parser() -> argparse.ArgumentParser:
         choices=SOURCE_MODES,
         default="auto",
         help="spectrum audio source: auto (monitor→stream→sim), monitor "
-        "(parec on Linux, an avfoundation loopback device on macOS), "
+        "(parec on Linux, a Core Audio process tap on macOS), "
         "stream (muted second ffmpeg decode), or sim",
     )
     parser.add_argument(