fix: normalise FLAC samples by 2^(bps-1) instead of 2^bps - #340
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Noticed every FLAC track plays about 6 dB quieter than the same track over Vorbis. The decoder divides by 2^bits_per_sample, but libFLAC's write callback delivers signed samples in native range, so converting to [-1, 1] needs 2^(bps-1). The current divisor halves everything: a full-scale 16-bit sine (peak sample 32767) decodes to 0.499985 where it should be 0.999969, the player's own maxSampleValue. The vorbis path gets floats straight from libvorbis with no divisor at all, which is why the two codecs differ by exactly a factor of two at the same gain. 24-bit is affected identically. Added a regression test with a small committed fixture, since nothing else measures output amplitude and this survived compile, CI and review unnoticed.