canbus: furnace + water-heater status sensors (running + DSI fault)

Decode node 89 (furnace) read-only: Furnace Running (page-3 b0 bit0) and
Furnace DSI Fault (b0 bit5), same type-0x1E DSI encoding as the water
heater. The furnace is thermostat-controlled, not a Lippert load — it
sits on the bus only to report ignition state. Captures only ever showed
0x80 (off), so the 0x81 running / 0xA0 fault bytes are inferred by
parallel to node 95; confirm on the first real burn.

Add Water Heater Heating (node 95 page-3 x[3] bit7 = burner actively
firing) — distinct from the on/off switch, which only reflects "enabled".

Carry all three through the MQTT bridge to home HA, add the furnace
DSI-fault Octavia alert mirroring the water heater, and update the README
node map (89 was "unknown switched load"). Campsite + home Camper
dashboards updated live.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

canbus/README.md

@@ -65,7 +65,7 @@ mapped**, and the **command path is implemented and confirmed by live actuation
 | `F8` | **interior lights** | type 0x1E; toggle test t≈51–61 s; **operated live 2026-06-12** ✓ |
 | `95` | **water heater** | type 0x1E; toggle test t≈85–94 s |
 | `61` | **water pump** | type 0x1E; toggle test 2026-06-11 (on 13.5s / off 23.8s) ✓ |
-| `89` | unknown switched load | type 0x1E, never toggled (furnace? DSI?) |
+| `89` | **furnace** (read-only) | type 0x1E; thermostat-controlled, not a Lippert load — present only to report DSI state. Same page-3 encoding as the heater: b0 bit0 = running, bit5 = DSI lockout. Stayed 0x80 (off) all bench captures; running/fault bytes inferred from node 95, confirm on first burn |
 | `75` | **awning** | type 0x21; jog test 2026-06-11 — b0 C0→C3 (in?) →C0→C2 (out?) with motor current on b2-3 |
 | `6A`, `7F`, `9C` | slide / jacks / movement class | type 0x21, untested |
 | `AE` | unknown (type 0x27, page3=0x00) | LP gas sensor? |