oven grabber: don't let isFrozen short-circuit the dud check
All-black captures are both "frozen" (frames identical to each other)
and "uniform-color" (no per-frame variance), so the previous logic
that skipped isUniformColorContent when isFrozen=true silently let
the broken bake fall through to the still-encoder. Always run the
uniform check; the isFrozen branch is reserved for legitimate frozen
art (identical-but-not-uniform frames).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>