fix(producer): render -c <scene> uses the scene's own duration, not the project's

[miguel-heygen]

Problem

Rendering a single sub-composition standalone —

hyperframes render -c compositions/scene.html

— renders it for the whole project's duration instead of the scene's own. A 2s scene mounted in a 12s project renders as 12s. When the project's master root derives its length from sibling mounts (rather than a literal data-duration), the same path instead throws:

[FrameCapture] Composition has zero duration

This is a false-negative against the documented QA flow: the scene snapshots and validates correctly standalone, so the wrong length only shows up in the final render.

Root cause

To render a scene standalone, the producer reuses the real index.html shell and isolates the matching mount (extractStandaloneEntryFromIndex → createStandaloneEntryRenderClone). It shallow-clones the master root and hangs only the scene's mount under it — but the shallow clone keeps the master's data-duration, so the extracted composition advertises the project length. The compiler reads that root duration verbatim.

Fix

Re-point the extracted wrapper's data-duration at the scene's own, read from the scene file's <template> root (the source of truth for that scene), with a fallback to the mount's data-duration. Full-project renders never take the extraction branch, so they're unaffected.

Verification

End-to-end via the pre-capture duration gate on a 3-scene project (master 12s; scene1 2s; scene2 10s):

render target	before	after
-c scene1 (own 2s)	12s ❌	2s ✅
-c scene2 (own 10s)	12s ❌	10s ✅
index.html (master)	12s ✅	12s ✅

Adds unit coverage for both the scene-file and mount-fallback paths.