feat: modern HUD dashboard + telemetry expansion (#43)

ui/index.html — full dashboard rewrite:
- 3-column layout: LEFT telemetry gauges, CENTER 3D SaltyBot, RIGHT comms
- LEFT: artificial horizon (canvas, pitch/roll/ladder/roll-arc), yaw compass
  tape, pitch/roll/yaw readouts, bidirectional motor bar, battery bar, BME280
  environment section (auto-shows on data), MAG heading row
- CENTER: Three.js SaltyBot model (PR#41) with ground plane + animated
  wheel rolling proportional to motor_cmd
- RIGHT: USB tx/rx packet counters, mode badge (MANUAL/ASSISTED/AUTO),
  CRSF RSSI/LQ, dual RC stick overlay canvases (CH1–4), Jetson active dot
- BOTTOM: KP/KI/KD/SP/MAX sliders with APPLY + QUERY, collapsible log console
- Style: Tailwind CSS CDN, dark cyberpunk theme, neon cyan + orange accents

src/main.c — telemetry JSON additions:
- buf: 256 → 320 bytes (headroom for new fields)
- ja: Jetson active flag (0/1) via jetson_cmd_is_active()
- txc: TX telemetry frame counter (uint32, main-loop local)
- rxc: RX CDC packet counter (cdc_rx_count from usbd_cdc_if)
- ch1–ch4: CRSF channels mapped to µs (1000–2000) via crsf_to_range(),
  appended alongside rssi/lq when RC is alive

lib/USB_CDC/src/usbd_cdc_if.c:
- cdc_rx_count: volatile uint32_t, incremented in CDC_Receive on every
  packet; extern'd in main.c for telemetry

Closes #43.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/USB_CDC/src/usbd_cdc_if.c

@@ -4,9 +4,10 @@
 extern USBD_HandleTypeDef hUsbDevice;
 volatile uint8_t cdc_streaming = 1;  /* auto-stream */
 static volatile uint8_t cdc_port_open = 0;  /* set when host asserts DTR */
-volatile uint8_t cdc_arm_request = 0;    /* set by A command */
-volatile uint8_t cdc_disarm_request = 0; /* set by D command */
-volatile uint8_t cdc_recal_request = 0;  /* set by G command — gyro recalibration */
+volatile uint8_t  cdc_arm_request = 0;    /* set by A command */
+volatile uint8_t  cdc_disarm_request = 0; /* set by D command */
+volatile uint8_t  cdc_recal_request = 0;  /* set by G command — gyro recalibration */
+volatile uint32_t cdc_rx_count = 0;       /* total CDC packets received from host */
 
 /*
  * PID tuning command buffer.
@@ -174,6 +175,7 @@ static int8_t CDC_Receive(uint8_t *buf, uint32_t *len) {
     }
 
 done:
+    cdc_rx_count++;
     USBD_CDC_SetRxBuffer(&hUsbDevice, UserRxBuffer);
     USBD_CDC_ReceivePacket(&hUsbDevice);
     return USBD_OK;

src/main.c

@@ -20,10 +20,11 @@
 #include <string.h>
 #include <stdio.h>
 
-extern volatile uint8_t cdc_streaming;      /* set by S command in CDC RX */
-extern volatile uint8_t cdc_arm_request;    /* set by A command */
-extern volatile uint8_t cdc_disarm_request; /* set by D command */
-extern volatile uint8_t cdc_recal_request;  /* set by G command */
+extern volatile uint8_t  cdc_streaming;      /* set by S command in CDC RX */
+extern volatile uint8_t  cdc_arm_request;    /* set by A command */
+extern volatile uint8_t  cdc_disarm_request; /* set by D command */
+extern volatile uint8_t  cdc_recal_request;  /* set by G command */
+extern volatile uint32_t cdc_rx_count;       /* total CDC packets received */
 
 /*
  * Apply a PID tuning command string from the USB terminal.
@@ -156,8 +157,9 @@ int main(void) {
      */
     safety_init();
 
-    char buf[256];
+    char buf[320];
     int len;
+    uint32_t tx_count = 0;   /* telemetry frames sent to host */
 
     IMUData imu;
     uint32_t send_tick = 0;
@@ -340,17 +342,27 @@ int main(void) {
                     }
                 }
                 if (safety_rc_alive(now)) {
-                    /* RSSI in dBm (negative), link quality 0-100% */
-                    n = snprintf(p, rem, ",\"rssi\":%d,\"lq\":%d",
-                        (int)crsf_state.rssi_dbm, (int)crsf_state.link_quality);
+                    /* RSSI, link quality, and CH1–CH4 mapped to µs (1000–2000) */
+                    n = snprintf(p, rem, ",\"rssi\":%d,\"lq\":%d"
+                        ",\"ch1\":%d,\"ch2\":%d,\"ch3\":%d,\"ch4\":%d",
+                        (int)crsf_state.rssi_dbm, (int)crsf_state.link_quality,
+                        (int)crsf_to_range(crsf_state.channels[0], 1000, 2000),
+                        (int)crsf_to_range(crsf_state.channels[1], 1000, 2000),
+                        (int)crsf_to_range(crsf_state.channels[2], 1000, 2000),
+                        (int)crsf_to_range(crsf_state.channels[3], 1000, 2000));
                     p += n; rem -= n;
                 }
-                n = snprintf(p, rem, "}\n");
+                /* Jetson active flag, USB TX/RX packet counters */
+                n = snprintf(p, rem, ",\"ja\":%d,\"txc\":%u,\"rxc\":%u}\n",
+                    jetson_cmd_is_active(now) ? 1 : 0,
+                    (unsigned)tx_count,
+                    (unsigned)cdc_rx_count);
                 len = (int)(p + n - buf);
             } else {
                 len = snprintf(buf, sizeof(buf), "{\"err\":%d}\n", imu_ret);
             }
             CDC_Transmit((uint8_t *)buf, len);
+            tx_count++;
         }
 
         status_update(now, (imu_ret == 0),