File ▸ Import and the Media Browser offer `.aac` (it is in AUDIO_EXTENSIONS),
and Export ▸ AAC Audio writes ADTS `.aac` (#814), but no opener read them:
`audio::opener` only accepted mp3/mp2/flac/ogg/aif by extension, so every
`.aac` failed with "unrecognised format".
`AudioFileSource` now decodes a standalone ADTS stream with our AAC decoder
(ISO/IEC 13818-7 §6.2): frame after frame from the first header (after an
ID3v2 tag), resyncing past damage, skipping frames with another
configuration, keeping a frame that doesn't decode as silence, and
upsampling HE-AAC to its output rate. Decoding stops with an error past
4 GiB of samples instead of allocating without bound.
Tests encode a stereo tone with our encoder's ADTS output and import it
(rate, channels, duration, level), and run truncated, bit-flipped and
bad-length streams under catch_unwind.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
