feat: render (ffmpeg proxy preview) + compile (declarative spec → draft)

[renezander030]

Adds the two highest-utility next commands on top of v0.9, both validated end-to-end before merge.

render — watch an edit without opening CapCut

A low-res ffmpeg proxy of the timeline: flattens the main video track (per-segment source trim + speed), mixes every audio-track segment, and with --burn-captions draws the text segments in. Explicitly a preview, not CapCut's final render (no multi-track compositing/effects). Read-only — never mutates the draft.

• The ffmpeg invocation is built by a pure, deterministic buildRenderPlan that is unit-tested without invoking ffmpeg.
• --dry-run prints that plan and needs no ffmpeg at all.
• ffmpeg is an opt-in shell-out, only when actually rendering — same pattern caption uses for whisper. Still zero npm-dep.

capcut render ./my-short --burn-captions       # -> ./my-short/preview.mp4
capcut render ./my-short --dry-run             # print the ffmpeg plan, run nothing

compile — build a whole draft from a declarative JSON spec

The inverse of describe: instead of chaining 30 mutating add-* commands (each a place to drift), an agent emits one spec and compile constructs the draft atomically via the same factory functions the imperative commands use.

• Times in seconds (converted to CapCut microseconds); media paths resolve relative to the spec file.
• Validates the full spec and checks every media file exists before writing anything — a bad spec fails clean.
• Mirrors the result to both draft_content.json and draft_info.json so every downstream command reads the same data.

capcut compile ./short.json --out ./my-short

Verification

• 171 tests pass, 0 fail (12 new: 6 compile, 6 render incl. a real ffmpeg render gated on host availability + a pure-plan suite).
• compile output passes capcut lint (cross-tool validity check).
• Pre-commit gate (biome + full npm test) green.
• Two real bugs found and fixed during verification: compile was writing to draftsDir/spec.name instead of --out (dir name vs draft display-name conflation), and the dual draft_content.json/draft_info.json template files are now kept consistent.

Bumps to v0.10.0; CHANGELOG + README updated.

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