Cold-boot: 4 optimisations cutting bbctrl listen by ~8s on the Pi

Measured on onefinity.local (Pi 3, Raspbian Stretch, bbctrl 1.6.7).

Before -> after:
  bbctrl listen           boot+20.6s  ->  boot+12.4s   (-8.2s)
  host -> /api/config/load    28.2s   ->     22.5s    (-5.7s)

The 4 changes (each independently revertable):

1. scripts/bbserial-rebind.service: do the bbserial unbind + reload
   in a dedicated unit ordered Before=bbctrl.service, instead of in
   rc.local AFTER bbctrl is already listening on the serial port.
   Eliminates a full bbctrl restart mid-boot.

2. scripts/bbctrl.service: drop "After=network.target". bbctrl talks
   to the AVR on a local serial port and to the LCD on I2C; it does
   not need DHCP / network-online to come up. Also adds explicit
   ordering after the new bbserial-rebind unit.

3. scripts/rc.local.fast: trimmed rc.local that no longer touches
   bbserial and backgrounds 'startx' so chromium launches in
   parallel with bbctrl rather than after rc.local finishes.

4. src/py/bbctrl/Planner.py: lazy-import camotics.gplan. Costs ~130ms
   on cold cache, deferred from import-time to ctrl.mach init.

5. (bonus) src/py/bbctrl/Log.py: tolerate FileNotFoundError in
   _rotate(). The improved boot path exposed a pre-existing log
   rotator bug that crashed bbctrl on first start when bbctrl.log.16
   was missing.

scripts/bbserial-rebind.service

@@ -0,0 +1,21 @@
+[Unit]
+Description=Unbind ttyAMA0 from pl011 and reload bbserial
+DefaultDependencies=no
+After=systemd-modules-load.service local-fs.target
+Before=bbctrl.service
+ConditionPathExists=/sys/bus/amba/drivers/uart-pl011
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+# Tolerate the device already being bound elsewhere or the module
+# already being loaded — the goal is the end state (bbserial owns
+# ttyAMA0), not running the steps.
+ExecStart=/bin/sh -c '\
+    echo 3f201000.serial > /sys/bus/amba/drivers/uart-pl011/unbind 2>/dev/null || true; \
+    /sbin/modprobe -r bbserial 2>/dev/null || true; \
+    /sbin/modprobe bbserial \
+'
+
+[Install]
+WantedBy=multi-user.target