MagTag SSD1680 FPC-7519rev.b: tune VCOM to 0x14 (-1.0V)

Testing across 0x28→0x20→0x1c→0x18→0x14→0x10 confirmed 0x14 as the
sweet spot: background matches the FPC-7619rev.b panel without the dark
halo fringe artifacts that appear below 0x14.

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