Conflicts:
- viewer.rs: the undecodable-footage notice moved into phase_scene on main; it now
collects its layers in footage_problems, which keeps the proxy and collapsed-precomp
rules of #1701.
- project.rs: main's row layout, with the problem marker going by item_problem.
- movie_bytes.rs: the H.264 pin is FilmCraft 09aaf9b's bytes plus main's 19-byte
BT.709 colr box (104_322 + 19 = 104_341).
render/footage.rs imports ItemKind now that main names its imports.