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>

components/aes67_tx/include/aes67_tx.h

@@ -19,3 +19,6 @@ esp_err_t aes67_tx_init(void);
 esp_err_t aes67_tx_start(void);
 // Set the audio source. NULL = built-in 1 kHz test tone at -18 dBFS, phase-locked to PTP time.
 void aes67_tx_set_source(aes67_tx_read_cb_t read);
+// Ready-made source: pink noise at -18 dBFS RMS (peaks about -6 dBFS), same on L and R.
+// Pass it to aes67_tx_set_source(). Keeps state: use for one stream only.
+size_t aes67_tx_pink_noise(int32_t *buf, size_t frames);