Add LilyGo T-Watch S3 variant

[djkazic]

ESP32-S3 + SX1262 companion-radio (BLE) support for the LilyGo T-Watch S3:

• AXP2101 PMU brought up via XPowersLib (enables the LoRa, display and sensor power rails)
• ST7789 240x240 display and FT5x06/FT6x36 capacitive touch via LovyanGFX
• on-device UI driven by touch, as the watch has no navigation buttons

Shared changes, both guarded/no-op for existing boards:

• DisplayDriver: add virtual getTouch() (default returns false; LGFXDisplay already implements it)
• companion ui-new: map touch taps to nav keys when UI_HAS_TOUCH is defined

Tested this with the official MeshCore app and was able to confirm that LoRA and BLE both work.

[djkazic]

I also have some power-saving tweaks locally, but was unsure if that was in scope for adding variant support. Let me know if I should push it.

More or less, it just defines a LOOP_IDLE_YIELD_MS var, and yields when idle so that the core can drop into waiti between events (rather than busy-spinning).