Replaces the encoder's 11 expect()/unreachable!() calls on internal invariants with handling
that can't panic: the rate-control state is passed in rather than re-borrowed, inter decisions
without a reference frame fall back to intra, per-plane results are bound directly, and the
writer's impossible branches become debug_asserts. Denies the panic lints in the crate.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
FilmCraft's AV1 decoder now frame-threads by default, so a picture may
come out of a later decode call; the round-trip check wants each
packet's picture from its own call.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>