Document planned battery (low) and Bluetooth (Matter joined) icon behavior

PROJECT_MEMORY.md

@@ -117,6 +117,17 @@ All compositions verified live on the panel.  (The earlier "no icon found"
 conclusion was wrong - the icons were in f[13] bits 3/4, which the first
 probe had not isolated.)
 
+### Planned status-icon behavior (for future battery + Matter work)
+
+- **Battery icon** (`f[13]` bit4, 0x10): to turn ON when the device battery
+  is **nearly empty** (low-battery warning).  Requires adding a battery/gas-
+  gauge circuit (e.g. ADC divider on VBAT, or an I²C fuel gauge) later.
+- **Bluetooth icon** (`f[13]` bit3, 0x08): to turn ON when the **Matter/
+  Thread network join/connection is successful**, OFF when not connected.
+  Requires the Matter cluster + OpenThread bring-up (see TODO).
+- Both compose with °C via OR: `f[13] = ICON_DEGC | (low_bat ? ICON_BAT : 0) |
+  (matter_ok ? ICON_BT : 0)` once those signals exist.
+
 ### Verified glyph segment map (from probing)
 
 For a digit byte-pair, the **first byte** = left column + horizontal bars, the
@@ -285,7 +296,8 @@ nix run .#flash
 - [x] **Find decimal point & % bits** — probing session mapped them: `f[4]/f[8]` bit5 = decimals, `f[10]` bit5 = %
 - [x] **Find Bluetooth & battery icons** — `f[13]` bit3 = Bluetooth, bit4 = battery; combos with °C work (0x0D/0x15/0x1D)
 - [ ] **Add humidity sensor** — wire up an external I²C sensor (SHT40, BME280, etc.) and show on display line 2
-- [ ] **Configure Matter/Thread** — enable OpenThread / Matter components in `sdkconfig`
+- [ ] **Add battery + low-battery icon** — battery circuit/gauge; drive `f[13]` bit4 (battery icon) when nearly empty
+- [ ] **Configure Matter/Thread** — enable OpenThread / Matter components in `sdkconfig`; drive `f[13]` bit3 (Bluetooth icon) on successful join
 - [ ] **Implement Matter cluster** — temperature and humidity measurement clusters
 - [ ] **Implement low-power operation** — deep sleep between measurements