MagTag SSD1680 FPC-7519rev.b: rename vcom20 → vcom14, add OTP note

Rename ssd1680_vcom20_display_start_sequence → ssd1680_vcom14_display_start_sequence
and DISPLAY_SSD1680_COLSTART_8_VCOM20 → DISPLAY_SSD1680_COLSTART_8_VCOM14 to match
the actual register value being set (0x14, not the earlier test value 0x20).

Add comment noting that VCOM=0x14 was independently confirmed by reading OTP register
0x2D on the physical panel — byte 1 of the response is 0x14, matching what the panel
manufacturer programmed as the recommended VCOM for this revision.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>