No headless browser is installed on an arm64 Linux hub, so check-app opens no page and the manual gets no pictures #109

On an arm64 Linux hub (the Compose deployment in a Colima VM on an Apple-silicon Mac) no headless browser is ever installed: CHROME_PLATFORMS in apps/hub/src/builds/starter.ts maps darwin-arm64, darwin-x64 and linux-x64 only, so sharedBrowser() returns nothing and every sandbox's $HOME/browsers stays empty.

Seen on 2026-11-02 on the Compose hub built from main at 59bc8c4: the coder's check-app reported

✗ the headless browser is installed
    nothing under $HOME/browsers

and never opened a page. Because no page was opened, no picture was saved under docs/manual/, the manual the coder wrote shows none, and the coder's browser tools cannot start either.

Chrome for Testing has published a linux-arm64 headless shell since 156.0.8078.4, the version the hub pins, at https://storage.googleapis.com/chrome-for-testing-public/156.0.8078.4/linux-arm64/chrome-headless-shell-linux-arm64.zip. The platform table should map linux-arm64 to it. The same entry was in #66, which was closed as superseded by #81; #81 did not carry it.