Round-3 cold-boot trims:
- mask sysstat.service (sadc CPU/IO logger; nothing reads it).
- mask dphys-swapfile.service and add /var/swap to /etc/fstab so swap
is brought up by systemd at local-fs.target instead of by an LSB
wrapper that re-checks the swap file size on every boot.
Both are reversible: `systemctl unmask <unit>` and remove the fstab
line. Before doing the dphys swap, install.sh verifies /var/swap
exists; on a fresh image where the file hasn't been created yet,
nothing is changed and dphys-swapfile keeps running normally.
Userspace boot 11.5s -> 10.7-11.4s on clean runs; bbctrl listen
unchanged at boot+10.4s (the saving moves to chromium/multi-user).
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).
