Replaces the encoder's 8 expect()/unwrap()/unreachable!() calls with handling that can't
panic: coding-unit decisions without a candidate report an infinite cost, inter helpers
without a reference picture return a neutral result, residual_coding of an all-zero block
writes nothing, the impossible Intra arms are no-ops, and the edge padding of an empty row uses
0 instead of unwrapping. Denies the panic lints in the crate.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
