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).

justfile

@@ -13,7 +13,7 @@ version := `sed -n 's/^version = "\(.*\)"/\1/p' Cargo.toml`
 # Xtensa targets are built with `+esp` + `-Zbuild-std=core,alloc` (the
 # self-contained esp toolchain ships its own rust-src; the public
 # nightly drifts ahead and breaks core-stdlib compilation).
-boards := "heltec_v3 heltec_v3_uart heltec_v4 elecrow_thinknode_m2 lilygo_tbeam lilygo_tbeam_supreme rak_wisblock_4631 waveshare_rp2040_lora wio_tracker_l1"
+boards := "heltec_mesh_node_t114 heltec_v3 heltec_v3_uart heltec_v4 elecrow_thinknode_m2 lilygo_tbeam lilygo_tbeam_supreme rak_wisblock_4631 waveshare_rp2040_lora wio_tracker_l1"
 
 # Install all required tools and toolchains
 setup:
@@ -246,6 +246,7 @@ _flash_uf2 board:
 [private]
 _info name:
     @case "{{name}}" in \
+        heltec_mesh_node_t114) echo "heltec_mesh_node_t114 thumbv7em-none-eabihf nRF52840_xxAA" ;; \
         heltec_v3)             echo "heltec_v3 xtensa-esp32s3-none-elf esp32s3" ;; \
         heltec_v3_uart)        echo "heltec_v3_uart xtensa-esp32s3-none-elf esp32s3" ;; \
         heltec_v4)             echo "heltec_v4 xtensa-esp32s3-none-elf esp32s3" ;; \