Phase 3 of the OS OTA pipeline. New module kiosk/src/os_update.rs polls
/api/kiosk/os/check with the kiosk's compatibility string and current OS
version (read from /etc/betterframe/os-compatibility +
/etc/betterframe/os-version, both written by the image build), downloads
the bundle, sha256-verifies the transport, and hands off to
`rauc install`. RAUC takes it from there: CMS signature verify against
/etc/rauc/keyring.pem, copy into inactive A/B slot, arm tryboot via the
custom bootloader backend, return. We then post /api/kiosk/os/applied
and `systemctl reboot` into the new slot.
Wired into the existing 60s heartbeat loop in ui.rs, gated by
BF_ENABLE_OS_OTA=1 (default OFF so dev kiosks on non-A/B images don't
keep trying + failing). Runs BEFORE the kiosk-binary check on each tick
so an OS bundle that ships an updated kiosk binary doesn't race the
firmware path.
On clean-boot heartbeat success we now also call `rauc status
mark-good` so the boot-attempts counter resets — three bad boots in a
row will auto-roll back without us needing a separate rollback path.
What's NOT in this commit:
- A/B partition layout in the pi-gen image (task #6, blocks actual
deployment — bundles can be served + accepted but `rauc install`
will refuse without two valid slots).
- Admin UI for managing releases + rollouts (task #4).
