feat: add design.md linter

[davideast]

Introduces the DESIGN.md linter. Follows a "Parse, don't validate" architecture where structural errors are surfaced as actionable diagnostics.

Public API

The primary entry point for the library is the lint() function. It accepts a raw YAML string (extracted from markdown or a standalone file) and returns a comprehensive report.

import { lint } from '@google/design.md';

const markdownResult = `---
name: Custom Theme
colors:
  primary: '#ff0000'
  accent: 'red' # Invalid color
---
# My Design System
`;

const report = lint(markdownResult);

// Check if the model was built successfully
console.log(report.designSystem.name); // "Custom Theme"

// Inspect diagnostics (both modeler errors and linter warnings)
report.diagnostics.forEach(diagnostic => {
  console.log(`[${diagnostic.severity.toUpperCase()}] ${diagnostic.path}: ${diagnostic.message}`);
});

// Quick summary counts
console.log(`Errors: ${report.summary.errors}`);
console.log(`Warnings: ${report.summary.warnings}`);
console.log(`Infos: ${report.summary.infos}`);

Advanced Usage: Custom Rules

You can also run the linter with a custom set of rules or pre-evaluate them for TUI applications:

import { runLinter, preEvaluate, DEFAULT_RULES } from '@google-labs-code/design.md-linter';

// Run with a subset of rules
const customResult = runLinter(designSystemState, [myCustomRule]);

// For interactive menus (groups by Fixes, Improvements, Suggestions)
const graded = preEvaluate(designSystemState);
console.log(graded.fixes); // Errors that must be fixed

Modeler-Emitted Diagnostics

The ModelHandler now exhaustively validates all properties during the resolution phase:

• Colors: Must be valid hex format (#RGB or #RRGGBB).
• Typography: fontFamily cannot be a color, fontWeight must be a number, and dimensions (fontSize, lineHeight, letterSpacing) are strictly restricted to px and rem units.
• Dimensions: rounded and spacing tokens are also restricted to px and rem.

Any invalid value is recorded as an error diagnostic rather than being silently dropped, ensuring the generated state is always valid.

Pluggable Linter Architecture

The linter has been refactored into a pure functional core with standalone rules:

• Rules: brokenRef, contrastCheck, missingPrimary, orphanedTokens, tokenSummary, and missingSections.
• Redundant rules (invalidColor, nonStandardUnit) were removed as they are now handled natively by the modeler's structural validation.