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/rc.local.fast

@@ -0,0 +1,40 @@
+#!/bin/bash
+# rc.local for the OneFinity Pi, "fast" variant.
+#
+# What changed vs. scripts/rc.local:
+#   - bbserial unbind/rebind moved to bbserial-rebind.service (runs
+#     once, before bbctrl, instead of after bbctrl is already
+#     listening on the serial port).
+#   - startx moved to kiosk.service so chromium starts in parallel
+#     with bbctrl rather than blocking on rc.local.
+#   - rc.local no longer keeps the Pi in 'starting' state forever,
+#     which fixes systemd-analyze.
+
+set -e
+
+# Mount /boot read only
+mount -o remount,ro /boot 2>/dev/null || true
+
+# Set SPI GPIO mode
+gpio mode 27 alt3 || true
+
+# Create browser memory limited cgroup
+if [ -d /sys/fs/cgroup/memory ]; then
+    CGROUP=/sys/fs/cgroup/memory/chrome
+    [ -d "$CGROUP" ] || mkdir -p "$CGROUP"
+    chown -R pi:pi "$CGROUP"
+    echo 650000000 > "$CGROUP/memory.soft_limit_in_bytes"
+    echo 750000000 > "$CGROUP/memory.limit_in_bytes"
+fi
+
+# Stop boot splash; harmless if plymouth already gone.
+plymouth quit 2>/dev/null || true
+
+# Start X (chromium kiosk) in the background so rc.local can exit and
+# late-boot units (bbctrl logrotate, etc.) don't block on it. Output
+# is redirected so the journal doesn't fill up with X warnings.
+cd /home/pi
+nohup sudo -u pi startx >/var/log/onefin-x.log 2>&1 &
+disown
+
+exit 0