feat: pageFrame validation + color policy

[mikeylong]

Changes

Implements two major features:

1. Layout.pageFrame Static Analysis Validation

• Validates pageFrame container existence and properties via static analysis
• Checks maxContentWidth, padding (left/right), and selector support
• New documentation: layout-pageframe.md
• Test fixtures and comprehensive test coverage

2. Color Policy + allowedColors Deprecation

• New color policy system with warn, strict, and off modes
• Denylist support (takes precedence even when policy is off)
• Deprecation warnings for legacy allowedColors field
• Traceable findings with location metadata
• New documentation: color-policy.md

Commits

• b29485f - feat: validate layout.pageFrame via static analysis
• 46d7167 - feat: add color policy + allowedColors deprecation + traceable findings

Testing

• Color-policy tests: 8/8 passing ✅
• PageFrame validation tests passing
• Build succeeds