feat(export): full-timeline video export orchestrator + export_video command

[Felix201209]

What

A clean, verifiable minimal slice of the export subsystem: a per-frame export orchestrator that connects two already-ready building blocks — the wgpu compositor (opentake-render) and the ffmpeg VideoEncoder (opentake-media) — into a pure-video (no audio) .mp4 export.

src-tauri/src/export.rs walks every frame of the current timeline:
plan.frame(f) → Compositor::render_to_rgba → DecodedFrame → RgbaFrame → VideoEncoder::push_frame, then finish(). Export renders at the full export resolution (export_render_size, not the preview cap).

Scope (first cut)

• H.264 / .mp4 only. H.265 (.mp4) and ProRes 422 (.mov) are accepted by the request type but rejected with a clear error until their container/preset branches are validated (TODO). The encoder already builds correct args for both; this command just hasn't opted them in.
• No audio mix, no progress callback / cancellation yet — deliberately deferred to keep the spine minimal and verifiable.
• Single-threaded serial composite (the Rc-textured compositor requires it; export is one frame at a time anyway).

Design / safety

• The manifest/text projection + MediaResolver + GPU-context logic is a self-contained copy of the preview path (render.rs), so composite_frame's body is untouched — zero collision with the in-flight playback/preview work (appergb 时间线合成预览 + 播放(wgpu composite_frame 接线 + 暂停态 canvas + 播放引擎) #47/时间线片段编辑收尾:选中/Delete 验证 + 切割 + 片段右键菜单(Copy/Swap Media/Save as Media/AI Edit) #48) or the media PRs (feat(media): folder browsing in media panel (#58) #77 / feat(media): extract audio track to local file (#39) #79).
• Orchestration is split into a Tauri-free run_export(timeline, manifest, project_dir, req) so it's drivable from an integration test without AppCore / Tauri State.
• Tauri boundary returns Err(String); opentake-domain stays zero-dependency; request DTO uses #[serde(default)] so a bare { "outPath": "..." } exports H.264/1080p.

Files

• New src-tauri/src/export.rs — orchestrator + export_video #[tauri::command].
• New src-tauri/tests/export_integration.rs — ffmpeg+GPU gated end-to-end test.
• src-tauri/src/lib.rs — 2 lines: pub mod export; + register export::export_video in invoke_handler.

Verification (all green on real hardware)

• cargo fmt --all — clean
• cargo clippy -p opentake-tauri --tests -- -D warnings — clean
• cargo test -p opentake-tauri --lib export — 6 unit tests pass (preset/codec/extension validation, unwired-codec rejection, serde defaults)
• cargo test -p opentake-tauri --test export_integration — 1 integration test passes: generates a 6-frame fixture, runs the full plan→composite→encode chain, and ffprobe confirms the output is a real 1280x720 h264 mp4 with exactly 6 frames. Auto-skips when ffmpeg/ffprobe or a GPU adapter are unavailable.

Refs SPEC §2.4 / §8.2.

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[appergb]

审核通过 ✅(自动审核流程 · 主控亲审)

• 新增 src-tauri/src/export.rs 整条时间线逐帧导出编排:snapshot→resolve_preset→project text/media→build_render_plan→逐帧 compositor.render_to_rgba→encoder.push_frame→finish。H.264/.mp4 限定,H.265/ProRes 明确拒绝并校验扩展名。run_export 与 Tauri 解耦便于测试。
• 资源/路径解析正确(External/Project 相对 project_dir)、source_frame.max(0)、tolerance=0 精确取帧、render_size 偶数化。
• 自包含复制 preview 逻辑、未碰 composite_frame → 与 #47/#48/#77/#79/#91 无碰撞(仅动 export.rs 新文件 + lib.rs 加 2 行注册 + 新集成测试)。CI 双绿。
• 测试齐全:单测 preset/quality/serde 默认 + ffmpeg/GPU 门控集成测试(真导出→ffprobe 校验宽高/编码/帧数,无 GPU/ffmpeg 自动跳过)。
• follow-up(非阻塞):run_export ~77 行 > 50 规约建议抽出帧循环;MediaResolver/projection 与 preview 路径重复,稳定后 hoist 为 pub(crate);out_path 缺路径边界校验(桌面 saveDialog 场景风险有限);H.265/ProRes/音频/进度取消按设计后续接。