Removes the six expect() calls from timeline export/import and denies the panic lints in the
crate. A precomp layer whose source item is not a composition (a damaged project, or an item
that was deleted) made export panic in Exporter::sequence; it now leaves that layer out.
Exporter::sequence and track_item return Option, the Motion/Opacity/Volume effect defaults
come from effect_or_new (None if FilmCraft's registry lacks one), and import's motion() skips
a layer without a transform group.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
