feat: add golden image diff helpers

[sjquant]

Why

Golden-image comparisons need to be deterministic enough for CI while still tolerating small rasterization differences across environments.

Changes

• add public perceptual-hash and pixel-metric comparison helpers with a CI assertion
• add quickthumb diff with text/JSON output, configurable tolerances, exit status, and optional visual diff output
• document the CLI and Python workflows for agent and CI pipelines
• add black-box coverage for exact matches, tolerated noise, mismatches, dimension errors, JSON output, and assertion failures