feat(ed047tc1): add partial_update() to reduce e-ink panel wear

[patrick3399]

Summary

• Adds partial_update(x, y, w, h, mode=MODE_GL16) for zone-specific refresh with selectable draw mode
• Extracts copy_buffer_to_epd_(x, y, w, h) helper to eliminate code duplication between update() and partial_update()
• update() behavior is unchanged — full screen, MODE_GC16

Draw mode guide

Mode	Flash	Grayscale	Wear	Recommended use
MODE_GC16	yes	16 levels	high	Full refresh, ghosting cleanup
MODE_GL16	no	16 levels	~3x lower	Routine partial updates (default)
MODE_DU	no	B&W only	lowest	Animation

Usage example

time:
  - platform: homeassistant
    on_time:
      - seconds: 0
        then:
          - lambda: |-
              id(my_display).partial_update(0, 0, 540, 560, MODE_GL16);

interval:
  - interval: 1h
    then:
      - component.update: my_display  # full GC16 to clear ghosting

Test plan

• Confirm partial_update() refreshes only the specified region
• Confirm update() still does full screen GC16 refresh
• Confirm no regression on M5Stack PaperS3

Closes #10