feat: add tiled contrast diagnostics

[sjquant]

Why

Low-contrast text diagnostics should fail on busy backgrounds when rendered text pixels are unreadable, even if the whole text region averages to a readable color.

Changes

• Add an internal reusable tiled contrast measurement helper.
• Use the worst text-pixel tile as the low-contrast diagnostic's contrast value while preserving threshold and adding structured tile metadata.
• Avoid scoring empty whitespace inside text bounds and keep rich-text colors tied to the pixels where each run renders.
• Cover mixed backgrounds, whitespace gaps, readable split-color rich text, default text color, and JSON lint output.