aes67_tx: mono sum; the source always delivers stereo

The pull callback's contract said "stream channels", but the player always
delivers stereo, so a 1-channel stream went out as L,R,L,R at half speed.
Sources now always deliver AES67_TX_SRC_CHANNELS (2); TX maps to the stream:
2 ch as is, 1 ch = (L+R)/2 in 64 bit with mono_sum (can't clip), else L.
Checked: mono packets 156 B, 1000/s, 48 samples each, SDP L24/48000/1.

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

main/player.c

@@ -18,7 +18,7 @@
 #include "spotify.h"
 
 #define RATE            48000
-#define CHANNELS        2
+#define CHANNELS        AES67_TX_SRC_CHANNELS   // the ring holds what TX pulls: stereo
 #define RING_FRAMES     (4 * RATE)      // 4 s in PSRAM
 #define PREFILL_FRAMES  (RATE)          // 1 s before (re)starting output
 #define FADE_FRAMES     (RATE * 30 / 1000)  // 30 ms ramp at a source switch