Trimmed music, asset links and legacy H.264 fixed

Four fixes, all found while building real videos for the blog.

• A trimmed clip with "length": "end" renders. A music bed with a trim and "length": "end" played its whole trim span instead of stopping at the end of the video, and the render failed as incomplete. A trimmed clip now starts at trim.from and plays for its length, never past trim.to.
• asset:// works inside a compose timeline. Clip sources written as asset:// or storage:// reached the renderer unresolved and failed to fetch. They now resolve like every other input, and a reference to an asset your workspace does not own is refused when you submit, before any credit is reserved.
• validate_compose warns about a font it cannot supply. A text clip naming a family we could not find rendered in a substitute without saying so. Validation now names the family and the fix: pick a Google Fonts family, declare the file in inputs.fonts, or add a fallback. It also catches a family spelled with the wrong capitals.
• compress.target with compatibility: "legacy" outputs H.264 Main. Legacy mode returned H.264 High with B-frames, which older decoders and the WhatsApp Business API reject. It now encodes H.264 Main with no B-frames, a level that fits the frame, 4:2:0 colour and AAC-LC audio, and always re-encodes.

Timelines are covered in the compose docs and size targets in the compress.target docs.