Add GOP-aware smart rendering to video concatenation

[JPPhoto]

Summary

#9833 laid the groundwork for adding GOP-aware smart rendering to video concatenation. This PR adds it to video_concat (version 1.3.0), preserving original H.264 video data outside transition regions when inputs are compatible. It re-encodes transition bridges extended to safe GOP boundaries and falls back to the existing full-encoding path when stream copying is unsafe. Existing transitions and audio assembly are retained.

Related Issues / Discussions

Builds on #9833.

QA Instructions

QA Instructions

• PYTHONOPTIMIZE=0 OMP_NUM_THREADS=1 uv run --no-sync pytest -p no:cacheprovider tests/app/invocations/test_video_concat_splice.py - 49 passed.
• Ruff 0.11.2 check and format checks passed for the seven changed Python files.
• Chromium playback: five seeks around splice boundaries succeeded.
• One warm-cache benchmark with two 40-second 640x360 clips at 30 FPS and an 8-frame crossfade: smart rendering copied 2,280 of 2,392 output frames and encoded 112 across one bridge in 3.74 seconds; full encoding took 6.46 seconds. Peak RSS was 1,419.2 MiB smart versus 1,453.5 MiB full.

Review

Added real-FFmpeg checks comparing decoded smart-render bridge frames with independent transition math and the full-encode path for crossfade and even/odd fades. The transition-value coverage finding is resolved.

Compatibility / Rollout

The video_concat invocation is version 1.3.0 and adds smart_render, enabled by default. Existing workflows remain compatible. Unsupported inputs and failed output validation fall back to full encoding; set smart_render to false to force that path.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Meaningful regression coverage added / updated where needed; obsolete tests/code removed
• Persisted-state and API changes include required migrations / compatibility validation
• Relevant performance/efficiency opportunities considered; material claims have evidence
• Material review findings resolved and relevant checks rerun
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

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