canbus: add DSI-fault capture plan to open items

Close propane valve, force a water-heater lockout, capture + diff against
healthy baseline. Prime suspects: node 95 b1 (always FF) or node AE page-3
(always 00). Whichever flips becomes the DSI binary_sensor.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

canbus/README.md

@@ -134,6 +134,20 @@ uses to build its UI.
 `6A`/`7F`/`9C` (movement — slide?), find battery SoC / the "4 green lights"
 source.
 
+### TODO: capture the DSI fault (planned 2026-06-12)
+
+The water-heater DSI fault is almost certainly on the bus but every capture so
+far is of a *healthy* heater, so the fault encoding is unknown. **Plan:** close
+the propane tank valve, run the water heater on gas until it locks out (DSI
+fault light on panel), capture ~20 s with the CANable, then diff against a
+healthy baseline. Prime suspects (both sit at a constant "all-clear" sentinel
+in current captures):
+- **node `95` (heater) page-3 `b1`** — always `0xFF`; expect it to drop/clear a bit on fault.
+- **node `AE` (type 0x27, ?LP-gas/diagnostics) page-3** — always `0x00`; expect non-zero on fault.
+
+Whichever flips → becomes a `binary_sensor` in the ESPHome node (the DSI fault
+the BLE app never exposed). Reset = reopen valve, re-light.
+
 ---
 
 ## Hardware (BOM)