Add STEMMA QT wiring alternative to hardware runbook

Document the 4-wire STEMMA QT path (QT-equipped backpack revisions) as an
alternative to soldering the 5-pin breakout header, with standard QT wire
colors mapped to Pi pins.

Author: lee

HARDWARE_RUNBOOK.md

@@ -19,7 +19,8 @@ A 3D-printable housing (STLs + Fusion source, with its own license) lives in
 - The Pi (Zero W or Zero 2 W). If it's a plain **Zero W** / **Zero 2 W** the GPIO header holes
   are **bare** — you'll also solder on a 2x20 header (or wires). The **WH** variants (Zero WH,
   Zero 2 WH) already have the header.
-- 5 jumper wires (female–female if both ends have male header pins)
+- 5 jumper wires (female–female if both ends have male header pins) — or, if your backpack
+  has STEMMA QT sockets, a STEMMA QT–to–female-jumper cable instead (see Step 3)
 - Optional but nice: a multimeter, helping-hands/clamp
 
 ## Step 1 — Solder the display to the backpack
@@ -29,7 +30,7 @@ A 3D-printable housing (STLs + Fusion source, with its own license) lives in
    shape to the white outline silk-screened on the backpack.
 2. Flip it over and solder every pin on the back. The HT16K33 chip ends up on the back.
 3. Solder the supplied header pins (or wires) into the row of breakout pads on the edge of the
-   backpack.
+   backpack. (Skip this if you'll wire via the STEMMA QT socket — see Step 3.)
 
 (If the Pi needs a header, solder that on now too.)
 
@@ -78,6 +79,24 @@ six pins** of the Pi's header. Pin 1 is the corner with the **square** solder pa
 Double-check `Vin`→5V and `Vio`→3V3 are **not swapped** before powering on. To see the header
 laid out for your exact board, run `pinout` on the Pi.
 
+### Alternative: STEMMA QT cable (4 wires, no breakout header)
+
+Recent backpack revisions (mid-2023 and later) add two **STEMMA QT** sockets (JST SH 4-pin)
+on the back. If yours has them, plug in a STEMMA QT–to–female-jumper cable
+(e.g. Adafruit #4397) instead of soldering the 5-pin breakout header:
+
+| QT wire (standard colors) | → | Pi physical pin |
+|---|---|---|
+| Yellow (`SCL`) | → | **5** (SCL / GPIO3) |
+| Blue (`SDA`) | → | **3** (SDA / GPIO2) |
+| Black (`GND`) | → | **6** (GND) |
+| Red (`V+`) | → | **1** (3V3) |
+
+Only four wires: the red 3.3V line sets the I2C logic level *and* feeds an onboard regulator
+that generates the 5V LED rail, so nothing connects to the Pi's 5V pins and the digits still
+run at full brightness. (Optional: to keep LED current off the Pi's 3.3V regulator, also wire
+the `Vin` pad → pin 2 (5V); the board switches to the external supply automatically.)
+
 ## Step 4 — Power on and verify
 
 ```bash