NEXUS: fix VBAT scale, Port C pinout, and ADC docs

Address hardware testing feedback from daijoubu:
- VBAT_SCALE_DEFAULT 1100 → 320 (hardware-verified)
- Port C connector pinout: pin 3 is SDA/TX (PB11), pin 4 is SCL/RX (PB10)
- Clarify OG Nexus has no EXT-V input (Vin ADC only, unlike X/XR)
- Update verification checklist with confirmed test results

Co-Authored-By: Joshua Perry <josh@6bit.com>

Author: nuketownada

src/main/target/NEXUS/README.md

@@ -31,7 +31,7 @@ For the Nexus-X and Nexus-XR, use the `NEXUSX` target instead.
 | Internal ELRS RX | None | RP4TD-M on UART5 |
 | PINIO1 (RX power) | None | PC8 |
 | UART1 pins | PA9/PA10 | PB6/PB7 |
-| Voltage input | 5-12.6V | 3.6-70V |
+| Voltage sense | Vin ADC only (5-12.6V) | EXT-V input (3.6-70V) |
 | Servo outputs | 5 | 7 default (9 max) |
 | Rotorflight target | NEXUS_F7 | NEXUSX |
 
@@ -68,9 +68,12 @@ For the Nexus-X and Nexus-XR, use the `NEXUSX` target instead.
 
 | Channel | Pin | Divider | Usage |
 |---------|-----|---------|-------|
-| BUS | PC2 | 320 | External battery voltage (mapped as VBAT) |
+| BUS | PC2 | 320 | Vin rail, 5-12.6V (mapped as VBAT) |
 | BEC | PC1 | 160 | 5V BEC rail monitoring |
 
+Note: Unlike the Nexus-X/XR, the OG Nexus has no dedicated EXT-V
+high-voltage battery sense input. VBAT monitors the board input power.
+
 ### Connector Pinouts
 
 **Port A (UART4 - CRSF receiver):**
@@ -88,8 +91,8 @@ For the Nexus-X and Nexus-XR, use the `NEXUSX` target instead.
 **Port C (UART3 / I2C2):**
 1. GND
 2. 5V
-3. SCL/TX (PB10)
-4. SDA/RX (PB11)
+3. SDA/TX (PB11)
+4. SCL/RX (PB10)
 
 **ESC Header:**
 - Signal: PB6 (TIM4_CH1 PWM)
@@ -113,15 +116,17 @@ For a 5-channel elevon glider like the Kunai:
 - [x] MCU boots, LEDs active (PC14, PC15)
 - [x] USB CDC enumeration and iNAV CLI
 - [x] IMU (gyro + accel) detected and responding
-- [x] iNAV Configurator 9.0 connects, 3D model tracks board movement
-- [ ] Barometer (SPL06 on I2C1) — fixed bus assignment, needs retest
-- [ ] VBAT ADC (PC2) — fixed pin assignment, needs retest
-- [ ] BEC ADC (PC1) — needs test
-- [ ] Gyro alignment (CW90) — fixed, needs retest
-- [ ] All servo outputs — need test
-- [ ] CRSF receiver on UART4 — needs test
-- [ ] Blackbox logging — needs test
-- [ ] VBAT scale calibration with multimeter
+- [x] Gyro alignment (CW90) confirmed
+- [x] Accelerometer working
+- [x] Barometer (SPL06 on I2C1) working
+- [x] VBAT ADC (PC2) working (scale 320)
+- [x] All UART ports (1-4, 6) verified
+- [x] UART4 CRSF receiver working (TX/RX swap confirmed)
+- [x] DShot on all motor outputs
+- [x] Servo outputs working
+- [x] Blackbox logging working
+- [x] LEDs working
+- [x] I2C2 bus working
 
 ## Building
 

src/main/target/NEXUS/target.h

@@ -129,19 +129,18 @@
 
 
 /* ---- ADC ---- */
-// From Rotorflight dump:
-//   ADC_BEC = PC1, divider 160 (BEC 5V rail monitoring)
-//   ADC_BUS = PC2, divider 320 (external battery voltage)
-// Map BUS voltage as VBAT for iNAV since that's the flight battery
+// OG Nexus has no EXT-V input (unlike X/XR which has a dedicated
+// high-voltage sense on PC0). Only two ADC channels:
+//   ADC_BUS = PC2, divider 320 (Vin rail, 5-12.6V)
+//   ADC_BEC = PC1, divider 160 (BEC 5V rail)
+// Map Vin as VBAT since it's the primary power input
 #define USE_ADC
 #define ADC_INSTANCE            ADC1
-#define ADC_CHANNEL_1_PIN       PC2   // BUS voltage (external battery)
+#define ADC_CHANNEL_1_PIN       PC2   // Vin (input power rail)
 #define VBAT_ADC_CHANNEL        ADC_CHN_1
-#define ADC_CHANNEL_2_PIN       PC1   // BEC voltage (5V rail)
-// VBAT scale: Rotorflight vbus_divider = 320
-// iNAV scale = divider * ~3.44 (ADC ref / resolution factor)
-// Start with 1100, calibrate with multimeter
-#define VBAT_SCALE_DEFAULT      1100
+#define ADC_CHANNEL_2_PIN       PC1   // BEC 5V rail
+// VBAT scale: hardware-verified value (divider ratio ~320)
+#define VBAT_SCALE_DEFAULT      320
 
 /* ---- Sensors ---- */
 #define SENSORS_SET             (SENSOR_ACC | SENSOR_BARO)