wav_writer.nx

wav_writer.nx -- serialise 16-bit PCM WAV files (canonical 44-byte header + raw samples). Paired with wav_header.nx for the reader half of the roundtrip. Use cases: debug tones, sovereign test-signal generation, dumping buffers from a DSP pipeline during bring-up, tiny synth prototypes without external tooling. Produces the standard 44-byte canonical PCM header: RIFF [size] WAVE fmt [16] fmt_code=1 channels sample_rate byte_rate block_align bits_per_sample data [size] <samples> Samples are little-endian signed i16 in (-32768..32767). Caller owns the sample buffer; this module writes header + copies. Invariants: W1 Only 16-bit PCM produced today. 8 / 24 / 32 bit and float formats pending a future commit. W2 num_samples is PER-CHANNEL sample count (not total sample-frames across channels). Total bytes = num_samples * num_channels * 2. W3 Output buffer must be at least 44 + 2*channels*num_samples bytes or WAV_ERR_SHORT is returned.