The previous fix for orphaned chapter mdats assumed the chapter text
mdat was dedicated and derived its location from stco[0] - 8. In
audiobooks that co-locate chapter text at the start of the primary
audio mdat (stco[0] == audioMdat.offset + 8), that arithmetic lands
on the audio mdat header, the "mdat" signature check passes, and the
full audio payload gets removed -- shrinking a 484 MB audiobook to
5.4 MB.
Fix: resolve the chapter mdat by finding the top-level mdat whose
data range contains stco[0], then re-parse after the trak/tref
removals and confirm no other track's stco/co64 points into that
mdat before deleting it. Shared mdats are left intact; the dead
chapter text bytes remain as harmless padding.
Add a regression test that writes a chapter track, patches its
stco[0] to point into the primary audio mdat (simulating the
audiobook layout), removes the chapter track, and verifies the
audio mdat is byte-identical afterwards.
85b6a9eb93