chantier: Remotion-parity — wave 2

[LeadcodeDev]

Tracking issue for the Remotion-parity chantier (2026-07-19), following the analysis comparing rustmotion's verified capabilities against Remotion 4.x video families. Same process as wave 1 (#24): one branch + squash-merged PR per workstream, sequential integration, full suite at each merge.

Execution order & status

Lot A — family unlockers

• feat(encode): extract and mix embedded video audio #44 feat(encode): embedded video audio — PR feat(encode): extract and mix embedded video audio #55
• feat(cli): props injection and batch rendering over the variables system #46 feat(cli): props injection + batch rendering — PR feat(cli): props injection and batch rendering over the variables system #56
• feat(fonts): Google Fonts fetching with local cache #50 feat(fonts): Google Fonts with cache — PR feat(fonts): Google Fonts fetching with local cache #57
• feat(captions): whisper transcription and SRT/VTT import #45 feat(captions): whisper transcription + SRT/VTT — PR feat(captions): whisper transcription and SRT/VTT import #58
• feat(audio): analysis pass and audio-reactive components #47 feat(audio): analysis pass + audio-reactive components — PR feat(audio): analysis pass and audio-reactive components #59

Lot B — motion-design finish

• feat(svg): progressive path draw-on animation #48 feat(svg): path draw-on — PR feat(svg): progressive path draw-on animation #60
• feat(engine): implement motion blur via temporal supersampling #49 feat(engine): motion blur (temporal supersampling) — PR feat(engine): implement motion blur via temporal supersampling #61
• feat(engine): scene-level post-processing effects #51 feat(engine): scene post-effects — PRs feat(engine): scene-level post-processing effects #62 + fix(engine): clean clippy lints from the post-effects merge #63

Backlog (instruction needed before commitment)

• feat(lottie): native Lottie decoding (evaluation) #52 feat(lottie): native decoding evaluation
• feat(engine): per-subtree time remapping #53 feat(engine): per-subtree time remapping
• Declared non-goal: 3D scenes/meshes (3D transforms + perspective cover the promo use cases; a 3D engine contradicts the Skia differentiator)

Decision log

• Whisper: subprocess-first (auto-detect whisper.cpp CLI, like the ffmpeg philosophy) — no heavy C++ build deps in the default binary; SRT/VTT import is fully offline.
• Audio analysis: rustfft (pure Rust) as a pre-analysis pass — single-binary principle intact.
• Squash-merge autonomous per the standing authorization (wave 1).

[LeadcodeDev]

Wave 2 complete: 9 workstreams squash-merged (PRs #55–#63), test suite 222 → 332, CI green at tip. Every Lot A family unlocker and all of Lot B shipped. Backlog issues #52 (Lottie evaluation) and #53 (time remapping) remain open — both deliberately instruction-gated. Declared non-goal recorded: 3D scenes/meshes.