Studio: selecting a non-root composition file in the Comps panel keeps showing the master composition #3543

Version: hyperframes 0.8.17 (via npx hyperframes@latest), WSL2 Ubuntu 22.04, Chrome.

Setup: a project with index.html (591 s, data-composition-id="main") plus parts/part-1.html … parts/part-4.html (142–174 s each, unique data-composition-id values), all generated from one timeline. lint/check pass. hyperframes render -c parts/part-1.html renders the correct 142 s video.

Repro:

1. hyperframes preview, open the project in Studio.
2. In the Comps panel, click parts/part-1.

Expected: the canvas and timeline show parts/part-1.html (142 s, its own splash/hook).

Actual: the row highlights and the hash becomes #project/<id>?comp=parts%2Fpart-1.html, but the canvas and timeline keep showing index.html (09:51, the root's splash). Same for every part. The part's thumbnail in the Comps panel is correct.

Notes from digging:

• Server side looks fine: GET /api/projects/<id>/preview/comp/parts/part-1.html returns the compiled part page with the right duration and content.
• In the Studio bundle, the preview iframe is only swapped to another file when drilling into a data-composition-src mount (if (!m.compositionSrc) return); a standalone .html chosen from the list never gets its own preview, so activeCompPath changes but the master stays mounted.
• Reproduced headlessly (puppeteer): after clicking the part, the timeline still reads 00:00 / 09:51 and the canvas shows the root hook sticker.

Impact: multi-part builds (one source → root + parts/part-N.html, rendered with render -c) can't be reviewed in Studio, and any edit made there lands in the root file instead of the part. Seen on two projects now.

Workaround: review parts via snapshot (temporarily swapping the part into index.html) or by rendering.