Pass detection preferences to M5GFX and stop assuming rejected boards

[ainyan03]

Summary

Follows the M5GFX board detection rework (requires m5stack/M5GFX#345).

• cfg.fallback_board is passed to M5GFX through Display.setDetectConfig() before Display.init(), so a provisional detection result that differs from it does not build the wrong display.
• A positive BOARD_ID is now an accepted, fixed firmware identity: M5GFX starts that board directly (no other families, no NVS). M5Unified selects it first and keeps it even when display startup fails. With a fixed BOARD_ID, the early ESP32-S3 GPIO46 hold is applied only when that board uses GPIO46 for power hold.
• The package default (used only when nothing was detected) is restricted to boards that detection cannot reject:
  • ESP32 D0WDQ6: TimerCam → unknown (TimerCam has its own detector)
  • other ESP32 packages without M5 boards: AtomLite → unknown
  • ESP32-C6 / ESP32-H2 / ESP32-C5: NanoC6 / NanoH2 / StampC5 → unknown. StampC6 and StampC5 arrive as M5GFX candidates instead
  • PICO-D4 (StampPico), PICO-V3 (AtomPsram), ESP32-S3, ESP32-C3 and ESP32-P4 defaults are unchanged
  • Arduino board-definition selections are unchanged

Behavior changes

• An unknown board does not get board-specific power hold, LEDs or buttons. On ESP32 the generic internal I2C (G21/G22) is still started.
• A TimerCam whose RTC does not answer no longer holds G33 by default (on battery it powers off). Use the TimerCam Arduino board definition, cfg.fallback_board or BOARD_ID.
• StampC6 / StampC5 are still selected, but no display object is added (getDisplayCount() 1 → 0).
• Generic ESP32 boards (e.g. D0WD dev kits) are no longer treated as AtomLite (no LED on G27, internal I2C on G21/G22 instead of G21/G25).

Testing

Built together with the M5GFX change on the M5Unified / M5GFX CI versions (Arduino and ESP-IDF), and checked on 41 devices. This PR's CI fetches M5GFX from the develop branch, so it fails until the M5GFX change is merged.