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/widgets/help.py

@@ -53,10 +53,11 @@ ABOUT_LINUX = (
 
 ABOUT_MACOS = (
     (
-        "The spectrum follows your audio output (SYNC) when a loopback device "
-        "such as BlackHole is available for ffmpeg/avfoundation; otherwise it "
-        "falls back to a muted ffmpeg stream tap (FFT, can drift) or a "
-        "synthesized display (SIM)."
+        "The spectrum captures your Mac's real audio output (SYNC) with a "
+        "native Core Audio process tap — nothing to install. macOS asks for "
+        "System Audio Recording permission the first time; without it the "
+        "analyser falls back to a muted ffmpeg stream tap (FFT, can drift) "
+        "or a synthesized display (SIM)."
     ),
     (
         "mpv publishes what is playing to the macOS Now Playing session, so "