Persist the cold-boot wins (was: only manually deployed via
tmp/20260501_restart_timing/deploy-fast.sh, would silently revert on
the next prod firmware update).
- Install bbserial-rebind.service alongside bbctrl.service and enable
it. Eliminates the rc.local bbserial reload mid-boot.
- Prefer scripts/rc.local.fast over scripts/rc.local when present.
Legacy rc.local left as a fallback for old firmware tarballs.
- Mask plymouth-read-write, plymouth-quit-wait, and raspi-config.
Together these were ~6s of userspace startup that bought nothing
on a deployed Onefinity Pi.
Cumulative: bbctrl listening at boot+10.6s (was 20.6s), userspace
boot 11.5s (was ~13s), bbctrl.service @2.9s in critical-chain (was
@6.5s after the first optimisation pass).
