Pack PCM into uncompressed ALAC frames for AirPlay/RAOP.

Real senders send compressed ALAC, but every RAOP receiver also decodes the uncompressed/"escape" form (a per-frame flag), so we avoid needing an ALAC compressor: we emit verbatim frames the receiver decodes identically.

Bit layout (MSB-first), confirmed against shairport's hammerton decoder: for 16-bit stereo the frame is 001 (ID_CPE), a 4-bit element-instance tag, 12 unused bits, partialFrame(1)=0, bytesShifted(2)=0, escape(1)=1 (uncompressed), then frames×(L:16, R:16) sample bits, then 111 (ID_END), zero-padded to a byte. The 4-bit instance tag matters: without it the decoder's partialFrame flag lands in the sample data and it reads a garbage sample count (silence still decodes — its sample bits are 0 — but real audio overflows).