M3: display driver (Pi-style DSI panel: TC358762 + ATTINY MCU)

display.{h,c} brings up the Hosyond 5\" 800x480 panel via the Raspberry
Pi 7\" touchscreen architecture:

- I2C init to the panel-side MCU at 0x45 (Pi 7\" register map ported
  from drivers/gpu/drm/panel/panel-raspberrypi-touchscreen.c).
- POWERON write + 120ms settle.
- ESP32-P4 MIPI-DSI bring-up in DPI video mode: 2-lane, 480 Mbps,
  800x480 @ 60Hz, RGB565, canonical Pi 7\" timings (HSW=18/HBP=20/HFP=62,
  VSW=4/VBP=27/VFP=18, 30 MHz pixel clock).
- Gamma-corrected (^2.2) PWM brightness via REG_PWM (0x86).
- display_sleep / display_wake hooks for M6.
- display_fill() and display_test_pattern() (8 vertical color bars)
  for M3 acceptance and reuse by M8 local screens.

app_main calls display_init() as best-effort — if the panel isn't
attached or wiring is wrong, the rest of the firmware (Ethernet, mDNS,
/state) keeps running. On success it paints the test pattern.

Pin assignments (PIN_I2C_SDA=7, PIN_I2C_SCL=8) match Waveshare's
bundled-panel BSP convention but are flagged TODO until the user's
specific FPC adapter wiring is confirmed.

main/CMakeLists.txt: add driver, esp_driver_i2c, esp_lcd to REQUIRES.

Build verified clean against ESP-IDF 5.4 for target esp32p4. M3
acceptance (color bars on screen) cannot be verified without a
flashed board + connected panel.

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