feat(t114): add Heltec Mesh Node T114 board support

Adds nRF52840 + SX1262 + 1.14" 240×135 ST7789 color TFT support. First
color display board in the project; introduces a BoardDisplay trait so
each board renders to its native pixel format and resolution without
aliasing or color-quantization round-trips.

Architecture:
- New BoardDisplay trait (src/display.rs) bundles render_*, present(),
  set_bright/set_dim. Replaces the old DrawTarget<Color=BinaryColor>
  + DisplayBrightness combo as the LoRaBoard::DisplayDriver bound.
- Mono OLED drivers (Sh1106, Ssd1306Async) get adapter impls that
  delegate to the existing display::render module — no behavior change
  for any existing board.
- T114 ST7789 driver (src/driver/st7789.rs) wraps lcd-async (async fork
  of mipidsi) with a 64,800-byte raw byte framebuffer in .bss. Renders
  via lcd_async::raw_framebuf::RawFrameBuf<Rgb565, _>, presents via a
  single async DMA blit (panel.show_raw_data(...).await) — the await
  yields the executor for the ~65 ms SPI transfer so the radio task
  isn't starved during repaints.
- PWM backlight on PWM0 / P0.15 (active-LOW, inverted polarity) for
  smooth dim/bright transitions instead of GPIO toggle.
- Native 240×135 Rgb565 renderer (src/display/render_color.rs) with
  splash + dashboard + sparkline.
- Shared QR-code blit (src/display/qr.rs), generic over PixelColor,
  re-centers the 64×64 bitmap which has cells in cols 0..50 and ~14 px
  of right-side padding — both mono and color splash now show the QR
  with 7 px quiet zones on all four sides (was visibly left-shifted).
- Shared content types (BoardInfo, DashboardCtx) decouple what to draw
  from how each board renders it.
- create_display() now takes the board's full DisplayParts bundle
  (was just the I2C bus) so non-I2C displays can carry SPI + control
  pins through.

Hardware:
- VEXT_ENABLE on P0.21 driven HIGH at boot — powers the SX1262 + TFT
  rail. Without this, lora-phy init succeeds at the SPI level but
  TX_DONE never fires.
- VTFT_CTRL on P0.03, active LOW; backlight on P0.15, active LOW.
- TFT SPI2 (SCK P1.08 / MOSI P1.09) at 8 MHz over EasyDMA. Async
  ExclusiveDevice from embedded-hal-bus (with the 'async' feature).
- lcd-async builder at display_size(135, 240), display_offset(52, 40),
  Deg90 rotation, ColorInversion::Inverted — standard 1.14" ST7789
  die mapping.
- Linker: shared ld/nrf52840-memory.x (S140 SoftDevice preserved at
  0x01000-0x26000 per project policy).

src/hal/nrf52840.rs

@@ -13,11 +13,23 @@ bind_interrupts!(pub struct Irqs {
     USBD => embassy_nrf::usb::InterruptHandler<embassy_nrf::peripherals::USBD>;
     SPIM3 => embassy_nrf::spim::InterruptHandler<embassy_nrf::peripherals::SPI3>;
     TWISPI0 => embassy_nrf::twim::InterruptHandler<embassy_nrf::peripherals::TWISPI0>;
+    // SPIM2 is the second free SPI peripheral on nRF52840. Only used
+    // today by the Heltec T114 (ST7789 TFT panel on a write-only bus
+    // that can't share with the SX1262 on SPIM3 because the pins
+    // differ). Boards that don't use SPI2 don't construct anything
+    // against it and this binding is a no-op for them.
+    SPI2 => embassy_nrf::spim::InterruptHandler<embassy_nrf::peripherals::SPI2>;
 });
 
 // ── MCU-level types ─────────────────────────────────────────────────
 
 pub type SpiBus = embassy_nrf::spim::Spim<'static>;
+// I2C is used by some nRF boards (RAK4631 / Wio Tracker L1 OLED) but not
+// others (T114 uses SPI for the TFT). The `#[allow(dead_code)]` keeps
+// the type available for any board that wants it without per-feature
+// gating; rustc otherwise fires a dead-code lint when the active board
+// doesn't construct one.
+#[allow(dead_code)]
 pub type I2cBus = embassy_nrf::twim::Twim<'static>;
 pub type UsbNrfDriver =
     embassy_nrf::usb::Driver<'static, &'static embassy_nrf::usb::vbus_detect::SoftwareVbusDetect>;
@@ -37,6 +49,8 @@ pub fn share_spi_bus(spi: SpiBus) -> &'static embassy_sync::mutex::Mutex<NoopRaw
 ///
 /// Every nRF52840 Twim user needs a `&'static mut [u8]` buffer.
 /// This provides one via StaticCell so board files don't repeat the pattern.
+/// Unused on nRF boards that don't expose I2C (e.g. T114 uses SPI for the TFT).
+#[allow(dead_code)]
 pub fn alloc_i2c_buffer() -> &'static mut [u8; 256] {
     static TWIM_BUF: StaticCell<[u8; 256]> = StaticCell::new();
     TWIM_BUF.init([0u8; 256])