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).

Cargo.toml

@@ -40,6 +40,21 @@ static_cell = "2.1"
 display-interface = "0.5"
 embedded-graphics = "0.8"
 ssd1306 = { version = "0.10", features = ["async"] }
+# T114 ST7789 TFT (color, async SPI). Optional — only pulled in by the
+# heltec_mesh_node_t114 feature. Mono OLED boards keep their existing
+# display-interface + ssd1306/sh1106 stack and don't pay for these.
+#
+# `lcd-async` is an async fork of mipidsi: same model/builder shape but
+# `init()` and pixel writes are `async fn`s. Combined with embassy-nrf's
+# DMA-backed async Spim, the dashboard repaint yields the executor for
+# the duration of the SPI transfer instead of busy-looping for ~65 ms,
+# so the radio task isn't starved.
+#
+# embedded-hal-bus needs the `async` feature for `ExclusiveDevice` to
+# implement `embedded_hal_async::spi::SpiDevice` (in addition to the
+# blocking impl).
+lcd-async = { version = "0.1", optional = true }
+embedded-hal-bus = { version = "0.3", optional = true, features = ["async"] }
 
 # Async IO traits and combinators
 embedded-io-async = "0.7"
@@ -252,6 +267,20 @@ wio_tracker_l1 = [
   "embassy-executor/platform-cortex-m",
   "embassy-executor/executor-thread",
 ]
+heltec_mesh_node_t114 = [
+  "embassy",
+  "trace-defmt",
+  "dep:cortex-m",
+  "dep:cortex-m-rt",
+  "cortex-m-rt/set-vtor",
+  "dep:defmt-rtt",
+  "dep:embassy-nrf",
+  "dep:panic-probe",
+  "dep:lcd-async",
+  "dep:embedded-hal-bus",
+  "embassy-executor/platform-cortex-m",
+  "embassy-executor/executor-thread",
+]
 waveshare_rp2040_lora = [
   "embassy",
   "trace-defmt",