Establish measured visual-fidelity regression coverage

[emsi]

Problem

The renderer has targeted unit and GUI tests, but no deterministic suite compares representative native pages with trusted OneNote output. Layout behavior for overlap, nested lists and tables, font substitution, printouts, ink, and uncommon mathematics therefore lacks measured tolerances.

Acceptance criteria

• Redistributable fixtures and reference captures cover representative freeform layouts.
• The font, GTK, renderer, scale, and theme environment is reproducible.
• Object bounds, text baselines, clipping, and raster differences use documented tolerances.
• Failures produce useful bounded artifacts without exposing private notebook content.
• Expected platform variance and accepted deviations are explicit.

Content-specific defects remain tracked separately in #6 and #17.