Add the three cascade fields — tint, tint-mode, tint-lock — to the validated frontmatter shape for both _layout.md files and page frontmatter. Validates values, rejects invalid types/values with clear errors, distinguishes YAML null from missing per the cascade semantics.
7478366e515b1c3b924158df92a3tint?: string | null accepted in layout and page frontmatter; named tints validated against the configured tint registry where possible (warn rather than fail if an unknown tint is referenced, since tints may be added/removed at runtime)tint-mode?: 'auto' | 'light' | 'dark' accepted; invalid values produce a clear error ("tint-mode must be one of: auto, light, dark")tint-lock?: boolean accepted; non-boolean values rejected with a clear errornull (tint: ~ or tint: null in YAML) is preserved as JS null through parsing — distinguishable from missingundefined) — distinguishable from nullSchema extension in whichever module owns frontmatter parsing — likely packages/content/ or packages/types/. Most projects use a runtime schema validator; check what refrakt already uses.
The null vs missing distinction is the most error-prone part. Most YAML parsers handle it correctly by default, but the JS layer often coerces both to undefined. Write a specific test that parses both tint: (empty) and tint: ~ and confirms the parsed object distinguishes them.
The named-tint validation is advisory, not strict — a tint might be added in a plugin loaded after the page is parsed, or removed at runtime via config changes. Warn on unknown names; don't fail builds. The cascade resolution in WORK-212 will just pass the unknown name through; the engine in WORK-214 will set data-tint="unknown-name" which simply won't match any defined tint.
None (or minimal — independent of SPEC-048 since frontmatter parsing already exists). Can start in parallel with the foundation work.