Whether a footage file is there is an observation (footage.check sets
Footage::missing without an edit), not part of the footage. The
signature leaves it out, and keys saved by 0.22-0.24, which hashed it,
are recognised with the file either online or offline. Warp Stabilizer,
3D Camera Tracker and Roto Brush analyses are kept through the next edit
while the footage is offline and after it comes back.
analysis::frames_key writes what a layer's frames are made of field by
field: every struct and enum it writes is destructured or matched in full
and each variant is a fixed word, so a new field doesn't compile until it
is decided and a rename doesn't change keys. Names, labels, UI hints and
markers are left out. Roto Brush's Input Key writes masks and the effects
above it the same way, instead of their Debug text.
Keys saved by 0.22-0.24 are recognised by the old code, moved unchanged
to frames_key::legacy, and kept rather than rewritten (frozen mattes hash
them).
arnav-hiwarkar