fix(render): seek single-composition (-c) renders at the requested fps

[func25]

What

render -c <scene> -f 60 outputs a 60fps file whose motion only advances at 30fps (every other frame is a duplicate). The same scene in the full reel is fine. Verified it is not dedup and not VFR.

Why

A single <template> composition rendered with -c mounts under a synthetic wrapper root that has no timeline of its own; only the inner composition registers in window.__timelines. So:

1. The export poll waits for the root host's own timeline, times out, and the child timeline is never nested/seeked — the scene free-runs at the capture tick rate.
2. The deterministic seek quantizes to the runtime's default canonicalFps (30), not the export fps.

How

• runtime: when a wrapper root resolves a composite timeline but its host id has no registered timeline, publish it under that id so the poll resolves and the seek drives the nested children. No-op for full reels.
• engine: inject the export fps (window.__hfCanonicalFps) before page scripts so the runtime seeks on the export grid. Honors --fps.

Test plan

How was this tested?

• Unit tests added/updated
• Manual testing performed
• Documentation updated (if applicable)