docs: PROTOCOL.md tablet coordinate range (0..2199) and the device-code discriminator, confirmed against the running OS

reference/keyboard-mouse/PROTOCOL.md

@@ -217,6 +217,15 @@ only in device code (`0100`) and a 5-byte absolute-position report (header `110`
 **[confirmed]** (VCB02 App. D, §C.7.4; `linux-vsxxxaa.c` ABS branch.) Not needed for the level-1
 mouse test unless a tablet is what's attached.
 
+**Coordinate range:** the absolute position spans **`0..2199`** over the active area
+(VCB02 App. D.1). **[confirmed]**
+
+**How the terminal chooses** — there is no separate command and no setup record. The **device
+code in the `T` reply is the whole discriminator**, and the host then frames **3-byte relative**
+reports for a mouse (`0010`) or **5-byte absolute** ones for a tablet (`0100`). **[confirmed]**
+against the running OS in the harness: it sends `T`, reads `A0 04 00 00` from a tablet, and then
+reads RHRB in **groups of 5** — groups of 3 with a mouse.
+
 ---
 
 ## 5. How a device failure is reported back (both devices)