Pink noise as a source (mode "pink")

Core: aes67_tx_pink_noise(), a ready-made TX source: xorshift32 white noise
through Paul Kellet's pink filter, -18 dBFS RMS (peaks about -6 dBFS), same
on L and R. The player uses it like the tone (straight to TX, no volume or
gain). Checked on the board: -18 dBFS RMS, octave bands 63 Hz-4 kHz flat
within +-0.4 dB, L = R.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/index.html

@@ -66,6 +66,7 @@ e.g. curl -X POST http://p4-aes67.local/api/player/next
   <option value="hls">HLS / M3U8 URL</option>
   <option value="auto">Spotify, fail over to HLS</option>
   <option value="tone">Test tone (1 kHz, -18 dBFS)</option>
+  <option value="pink">Pink noise (-18 dBFS RMS)</option>
   <option value="off">Off (silence)</option></select></label>
 <label><span>Failover delay (s)</span><input name="source.failover_delay_s" type="number" min="0" max="3600"></label>
 <label><span>Also fail over when paused</span><input name="source.failover_on_pause" type="checkbox"></label>