The catalogue hands the visitor to the studio with a plain `<a href>`, and the
app has no router: following it threw the page away, so a scan that was halfway
through a roll died with it. A scan belongs to the tab, not to the screen that
started it.
The scan now lives outside the component (`scanSession`/`startScan`/`stopScan`,
watched by whoever is up), so the catalogue can unmount and come back to a
reading that never stopped; a screen that returns joins it and sees the toolbar
line, the stop button and the rows filling in. The internal links are taken over
for a history push *only while a scan is in flight* — everywhere else the
browser navigates exactly as before, so the landing-to-studio flow is untouched.
`scripts/scan-nav-check.mjs` is the regression: a roll read one frame at a time
off a slow server, handed to the studio mid-scan, back to the catalogue, and the
whole roll read to the end with the studio up, counting documents along the way.
3dtours
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