Use brightness-only desk sleep to fix black-screen wake

[zhangyibing001]

Summary

• Keep the CO5300 controller and shared AMOLED/audio/motor L3B rail powered
  during desk sleep.
• Set display brightness to zero without entering controller sleep or cutting
  the shared rail.
• Keep the existing panel power and reinitialization helpers available for
  recovery experiments.
• Document the higher standby-power tradeoff and update the C152 validation
  checklist.

Motivation

Physical C152 testing showed that the rail-off wake path could restore the
ESP32, BLE, touch, and button handling while leaving the AMOLED black. The
brightness-only path restores the display reliably because the controller and
shared rail never lose power.

Validation

• All six native logic tests pass.
• pio run -e m5stack-stopwatch succeeds.
• Build size: 55,024 bytes RAM (16.8%) and 1,308,765 bytes flash (20.0%).
• A combined image containing this change was flashed to a physical C152.
• A red-button desk-sleep cycle followed by a wake tap restored the display on
  the physical device; the failing rail-off image had woken input handling but
  left the display black.

Tradeoff

Brightness-only desk sleep consumes more standby power than disabling L3B. A
full long-duration battery measurement and the 100-cycle sleep/wake soak test
remain follow-up validation work.