OBD-II signals renamed obd_*, plus timing, trims, O2; GNSS signals; elm_connected; reboot command

Decision 30: CAN feeds displays, OBD-II (K-line via the ELM) calibrates, so every
obd2 signal carries the obd_ prefix and the plain names are left for decoded ones.
yukart/gnss/<signal> gives displays the fix as single values; gnss/fix stays for
navigation. Boundary: reboot beside poweroff, same guards.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

docs/GUIDELINES.md

@@ -9,7 +9,7 @@ type: guidelines
 - ⚠ **The unit appears in both the topic name and the payload, so they can
   contradict each other.** Generating both from one entry is what stops that; a
   hand-written `"unit"` reintroduces the failure it was meant to prevent.
-- Signal names carry their unit (`vehicle_speed_kph`, `coolant_temp_c`), so a unit
+- Signal names carry their unit (`gnss_speed_kph`, `obd_coolant_temp_c`), so a unit
   mismatch is visible at the call site rather than one registry lookup away.
 
 ## 2. Two clocks, and they answer different questions