Validate a composition body without saving it (dry-run)

Dry-run the FULL save-time authoring guard set against a Composition body WITHOUT persisting it and without a project. Runs the same pure, DB-free guards saveComposition enforces — wire shape, primitive membership (unknown_primitive / custom_missing_sourcecode), required-param shape (invalid_primitive_params), clip↔track family (clip_source_track_mismatch), reframe coverage (reframe_coverage_gap), and font admission (font_not_reproducible: a caption/param face that is neither bundled nor sealed in a brand font asset) — and returns ONE shape, 200 { valid, errors[] }, for BOTH wire-shape and deep-invariant failures (no 400-vs-200 split). valid:false lists every authoring error (sanitized envelopes) to fix before save/render. The ONLY checks NOT run here are the two DB-bound ones — trim-bounds (media duration) and image/asset-on-video-track (media kind) — which can only be caught at save/render.