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/driver/st7789.rs

@@ -0,0 +1,123 @@
+//! ST7789 240×135 IPS TFT driver wrapper for the Heltec Mesh Node T114.
+//!
+//! Wraps `lcd_async::Display<…, models::ST7789, …>` (async fork of
+//! mipidsi) with an in-RAM raw byte framebuffer (240×135 × 2 bytes
+//! Rgb565 BE = 64,800 bytes) and an active-LOW PWM backlight on PWM0.
+//!
+//! Renderers draw into the framebuffer via `lcd_async::raw_framebuf::RawFrameBuf`;
+//! `present()` awaits a single async DMA blit of the entire buffer to
+//! the panel via `panel.show_raw_data(...).await`. The await lets
+//! embassy-nrf's DMA-backed Spim yield the executor for the duration
+//! of the ~65 ms SPI transfer, so the radio task can run alongside
+//! and the SX1262 IRQ handler isn't starved during repaints.
+
+use embassy_nrf::pwm::{DutyCycle, SimplePwm};
+use embedded_graphics::pixelcolor::Rgb565;
+use lcd_async::interface::Interface;
+use lcd_async::models::ST7789;
+use lcd_async::raw_framebuf::RawFrameBuf;
+use lcd_async::Display;
+
+use crate::display::{render_color, BoardDisplay, BoardInfo, DashboardCtx};
+
+/// PWM countertop. Drives the backlight at ~1 kHz (well above flicker).
+pub const BL_MAX: u16 = 1000;
+
+fn bl_bright() -> DutyCycle {
+    // Inverted polarity: HIGH for 0 counts → always LOW → full bright.
+    DutyCycle::inverted(0)
+}
+
+fn bl_dim() -> DutyCycle {
+    // ~80% HIGH → ~20% LOW → 20% brightness.
+    DutyCycle::inverted((BL_MAX * 4) / 5)
+}
+
+#[allow(dead_code)]
+fn bl_off() -> DutyCycle {
+    DutyCycle::inverted(BL_MAX)
+}
+
+// ── Framebuffer ───────────────────────────────────────────────────────
+
+pub const FB_W: usize = 240;
+pub const FB_H: usize = 135;
+/// Bytes for one full Rgb565 frame (2 bytes/pixel, big-endian — matches
+/// what ST7789 expects in 16-bit color mode).
+pub const FB_BYTES: usize = FB_W * FB_H * 2;
+
+// ── St7789Color ───────────────────────────────────────────────────────
+
+pub struct St7789Color<DI, RST>
+where
+    DI: Interface,
+    RST: embedded_hal::digital::OutputPin,
+{
+    panel: Display<DI, ST7789, RST>,
+    backlight: SimplePwm<'static>,
+    fb: &'static mut [u8; FB_BYTES],
+}
+
+impl<DI, RST> St7789Color<DI, RST>
+where
+    DI: Interface,
+    RST: embedded_hal::digital::OutputPin,
+{
+    pub fn new(
+        panel: Display<DI, ST7789, RST>,
+        backlight: SimplePwm<'static>,
+        fb: &'static mut [u8; FB_BYTES],
+    ) -> Self {
+        Self { panel, backlight, fb }
+    }
+
+    /// Wrap the raw byte buffer as a `RawFrameBuf<Rgb565, _>` so
+    /// `embedded-graphics` primitives can draw into it, then run `f`.
+    /// The borrow ends when this returns, freeing `self.fb` for the
+    /// subsequent blit in `present()`.
+    fn with_fb<F: FnOnce(&mut RawFrameBuf<Rgb565, &mut [u8]>)>(&mut self, f: F) {
+        let mut fbuf = RawFrameBuf::<Rgb565, _>::new(&mut self.fb[..], FB_W, FB_H);
+        f(&mut fbuf);
+    }
+}
+
+impl<DI, RST> BoardDisplay for St7789Color<DI, RST>
+where
+    DI: Interface<Word = u8>,
+    RST: embedded_hal::digital::OutputPin,
+{
+    /// Blit the entire framebuffer to the panel via the async SPI
+    /// interface. embassy-nrf's Spim runs the transfer over EasyDMA;
+    /// the `.await` yields the executor for the duration so the radio
+    /// task can process SX1262 IRQs concurrently.
+    async fn present(&mut self) {
+        let _ = self
+            .panel
+            .show_raw_data(0, 0, FB_W as u16, FB_H as u16, self.fb.as_ref())
+            .await;
+    }
+
+    async fn set_bright(&mut self) {
+        self.backlight.set_duty(0, bl_bright());
+    }
+
+    async fn set_dim(&mut self) {
+        self.backlight.set_duty(0, bl_dim());
+    }
+
+    fn render_splash_learn_more(&mut self) {
+        self.with_fb(|fb| render_color::splash_learn_more(fb));
+    }
+
+    fn render_splash_info(&mut self, info: &BoardInfo<'_>) {
+        self.with_fb(|fb| render_color::splash_info(fb, info));
+    }
+
+    fn render_dashboard(&mut self, ctx: &DashboardCtx<'_>) {
+        self.with_fb(|fb| render_color::dashboard(fb, ctx));
+    }
+
+    fn render_blank(&mut self) {
+        self.with_fb(|fb| render_color::blank(fb));
+    }
+}