8-bit WAV is decoded as signed, but WAV 8-bit PCM is unsigned (silence reads as -128) #12

The bug

WAV 8-bit PCM is unsigned (0–255, silence at 128) per the spec. recode decodes it as signed, so every 8-bit sample comes back off by a 128 wrap:

raw payload bytes       [  0,  64, 128,  192, 255]
spec-correct amplitudes [-128, -64,   0,   64, 127]     # unsigned minus 128
recode decode_wav_bytes [   0,  64,-128,  -64,  -1]     # wrong

Digital silence (128) decodes as -128 — full negative.

Why it is only worth filing now

Before #11 the 8-bit path was usually reading from the wrong offset anyway: the old size-subtraction counted the RIFF word-alignment pad byte as header, so a 3-sample 8-bit file returned [2, 3, 0] where the payload was [1, 2, 3]. #11 fixed the alignment, so the 8-bit path now lands on the right bytes for the first time — which makes the sign error the remaining defect and newly reachable.

Why it was kept out of #11

The signedness is not decided in the WAV code. It comes from mk_pcm_audio_codec / num_find_num_type_for picking the struct character, whose blast radius is every recode consumer, not just the WAV path. Non-WAV PCM callers may legitimately want today's signed b.

Suggested shape

For width_bytes == 1 on the WAV path specifically, decode with the unsigned struct char B and subtract 128; the encoder adds it back. Leave generic PCM callers on the current signed behaviour, or give them an explicit signed= choice.

Acceptance test — the probe values above round-trip:

assert decode_wav_bytes(eight_bit_wav)[0] == [-128, -64, 0, 64, 127]
assert encode_wav_bytes([-128, -64, 0, 64, 127], sr, width_bytes=1) == eight_bit_wav

Note _wav(..., width=1) in test_recode.py already builds valid 8-bit fixtures.

Found by adversarial review while fixing #4.