fix: flac loses its tail at finalize (#497)

[remsky]

Output buffer is truncated after every chunk, so muxers that seek back to
patch a header at pos 0 on close (wav RIFF sizes, flac STREAMINFO, mp3 xing) were landing the patch on the flushed audio instead.

Finalize now reads before close for everything except ogg, which appends its last audio page during close. Drops the wav return b"" special case from #463, no longer needed with the correct order.

• wav/flac round-trip sample-exact, streaming and non-streaming payloads byte-identical
• mp3/aac/opus unchanged (verified byte-identical to master)
• regression test decodes and counts samples per format
• known limitation: streamed flac still carries total_samples=0 (inherent to streaming,
  spec-valid); libsndfile readers reject it, same as before this fix. ffmpeg/pyav decode fully.

Diagnosis by @Technologicat in #497.

[remsky]

Format	Single chunk (non-streaming)	Multi-chunk ×30 (streaming)	Payloads identical	PyAV	libsndfile
WAV	144078 B	144078 B	Yes	72000, sample-exact	72000, OK
FLAC	38009 B	38009 B	Yes	72000, sample-exact	Fails (pre-existing)
MP3	48812 B	48812 B	Yes	73152	73152, OK (encoder pad)
Opus	54983 B	54983 B	Decode-identical*	144000 @ 48 kHz = 3.000 s	72000, OK
AAC	26067 B	26067 B	Yes	73728 (encoder pad)	Fails (never supported)
PCM	144000 B	144000 B	Yes	Raw, sample-exact	N/A

* Opus bytes differ only in the random Ogg serial number stamped per writer; decoded output is identical.