Exclude CSS-wide keywords from font materialization plans

[chubes4]

Problem

FontMaterializationPlanBuilder currently accepts CSS-wide keywords as downloadable font families. The university fixture emits inherit beside real families in its Google Fonts materialization plan.

inherit, initial, unset, revert, and revert-layer are CSS control values, not typeface names. They should never enter provider requests, receipts, or SVG font evidence.

Acceptance criteria

• Exclude CSS-wide keywords during normalized font usage planning.
• Preserve legitimate quoted and multi-word family names.
• Add deterministic coverage for all CSS-wide keywords and existing generic/web-safe exclusions.
• Keep the font materialization schema unchanged.

Related parity work: #634 and #659.

AI assistance

• Model: OpenAI GPT-5.6 Sol
• Tool: OpenCode
• Used for: deterministic fixture reproduction and root-cause analysis with Chris Huber.