fix(skills): reject empty or partial scene files at assembly time

[miguel-heygen]

Summary

When a scene worker errors or is interrupted mid-write, it can leave an empty (or markup-less) compositions/<scene>.html. The assembler only checked that the file existed, so it still emitted a data-composition-src reference to it. The problem surfaced much later — at render — as the confusing compile error:

Composition HTML is empty or could not be parsed: compositions/scene-*.html

By then the broken project had already shipped to the user, and the message pointed at the symptom rather than the cause.

Changes

All three assemblers (product-launch-video, faceless-explainer, pr-to-video) now validate scene-file content — non-empty and contains markup — at the exact point they already read the file for the duration cross-check. If a scene file is blank or partial, assembly fails fast with an actionable message telling the author to re-dispatch that scene worker, before any reference to it is written into index.html.

This moves the failure from a late, opaque render error to an early, clear authoring error, and prevents an unrenderable project from being produced at all.

Verification

• node --check passes on all three scripts.
• Repro against the real assembler: empty and whitespace-only scene files now fail at assembly with the new message (exit 1); a valid scene file passes the guard and assembles successfully (no false positives).