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

schema/topics.json

@@ -12,7 +12,7 @@
       "payload": "envelope",
       "retained": true,
       "rate": "1-50/s",
-      "publisher": "decoder, obd-poller",
+      "publisher": "decoder (source z27ag), obd-poller (source obd2)",
       "description": "One topic per signal, named exactly as in signals.json."
     },
     "yukart/gnss/fix": {
@@ -22,6 +22,13 @@
       "publisher": "gnss",
       "description": "value is an object: lat, lon, alt_m, speed_kph, heading_deg, fix, satellites."
     },
+    "yukart/gnss/<signal>": {
+      "payload": "envelope",
+      "retained": true,
+      "rate": "1-10/s",
+      "publisher": "gnss",
+      "description": "The receiver's fix as single values, one topic per signal, named exactly as in signals.json (source=gnss). For displays; yukart/gnss/fix stays the composite for navigation."
+    },
     "yukart/host/<signal>": {
       "payload": "envelope",
       "retained": true,