feat: native USB CDC-ACM support for Heltec V3

The default heltec_v3 feature now uses native USB OTG (same as V4),
appearing as /dev/ttyACM0 with the DongLoRa VID:PID (1209:5741).
This requires a hardware mod: solder R29/R3 and disconnect the CP2102
from the USB data lines.

Stock (unmodified) V3 boards should use the new heltec_v3_uart feature
which preserves the original UART-over-CP2102 behavior (/dev/ttyUSB0).

- heltec_v3: switched from UART to USB OTG
- heltec_v3_uart: new feature, UART fallback for stock boards
- Removed #[cfg] gates on UsbParts/UartParts (both always defined)
- Updated README with hardware mod instructions and board table
- Updated justfile with heltec_v3_uart board definition

firmware/src/board/heltec_v3_uart.rs

@@ -0,0 +1,75 @@
+//! Heltec WiFi LoRa 32 V3 — stock UART (via CP2102 bridge).
+//!
+//! This is for unmodified V3 boards where USB goes through the CP2102
+//! bridge to UART0. The board appears as /dev/ttyUSB* (not ttyACM*).
+//!
+//! If you've done the hardware mod (solder R29/R3, disconnect CP2102),
+//! use the `heltec_v3` feature instead for native USB CDC-ACM support.
+
+use esp_hal::gpio::{Level, Output, OutputConfig};
+
+use super::esp32s3;
+use super::traits::{BoardParts, LoRaBoard};
+use crate::hal::esp32s3 as mcu;
+
+#[allow(unused_imports)] // Re-exported for other modules (display, radio, host tasks)
+pub use super::esp32s3::{
+    create_display, DisplayDriver, DisplayI2c, DisplayParts, RadioDriver, RadioParts, UartDriver,
+    UartParts,
+};
+
+pub struct Board {
+    p: esp_hal::peripherals::Peripherals,
+}
+
+impl LoRaBoard for Board {
+    const NAME: &'static str = "Heltec V3 (UART)";
+    const TX_POWER_RANGE: (i8, i8) = (-9, 22);
+
+    type RadioParts = RadioParts;
+    type CommParts = UartParts;
+    type DisplayParts = DisplayParts;
+    type DisplayDriver = DisplayDriver;
+
+    fn init() -> Self {
+        let p = esp_hal::init(esp_hal::Config::default());
+        Self { p }
+    }
+
+    fn mac_address() -> [u8; 6] {
+        mcu::mac_address()
+    }
+
+    fn into_parts(self) -> BoardParts<RadioParts, UartParts, DisplayParts> {
+        let p = self.p;
+
+        mcu::start_timer(p.TIMG0);
+
+        // Vext power: GPIO36, active LOW to enable peripherals
+        let vext = Output::new(p.GPIO36, Level::Low, OutputConfig::default());
+        core::mem::forget(vext);
+
+        let spi_bus = mcu::init_spi(p.SPI2, p.DMA_CH0, p.GPIO9, p.GPIO10, p.GPIO11);
+        let radio = esp32s3::init_radio(spi_bus, p.GPIO8, p.GPIO12, p.GPIO14, p.GPIO13);
+        let host = UartParts {
+            driver: mcu::init_uart(p.UART0, p.GPIO43, p.GPIO44),
+        };
+
+        // SSD1306 display reset: pulse GPIO21 low->high before I2C init
+        let mut display_rst = Output::new(p.GPIO21, Level::Low, OutputConfig::default());
+        esp_hal::delay::Delay::new().delay_millis(10);
+        display_rst.set_high();
+        esp_hal::delay::Delay::new().delay_millis(10);
+        core::mem::forget(display_rst);
+
+        let i2c = mcu::init_i2c(p.I2C0, p.GPIO17, p.GPIO18);
+        let display = Some(DisplayParts { i2c });
+
+        BoardParts {
+            radio,
+            host,
+            display,
+            mac: Self::mac_address(),
+        }
+    }
+}