Implement compact variable format

[wspringer]

Fixes #34

Summary

This PR implements a new compact variable format that replaces the verbose array-based approach with self-contained variable objects, addressing the verbosity issues identified in issue #34.

Key Changes

Breaking Change: New Variable Format

• Old format (removed): Separate arrays for bounds, types, and names
• New format: Array of self-contained variable objects with optional properties

Example Migration

// Before (verbose)
{
  "variables": {
    "bounds": [{"lower": 0, "upper": 100}],
    "types": ["continuous"], 
    "names": ["profit"]
  }
}

// After (compact)
{
  "variables": [
    {"name": "profit", "ub": 100}
  ]
}

Benefits

• Eliminates array synchronization errors - no more mismatched array lengths
• Reduces verbosity with smart defaults (lb=0, ub=+∞, type="cont")
• Industry standard abbreviations - lb/ub for bounds, cont/int/bin for types
• Self-contained - each variable object contains all its properties

Areas Requiring Verification

🔍 Schema Changes (src/schemas.ts)

• New CompactVariableSchema with optional properties
• Validation logic updated for array length checking
• Please verify the smart defaults and validation rules are correct

🔍 Processing Logic (src/lp-format.ts, src/index.ts)

• Updated to access variables[i].name instead of variables.names?.[i]
• Smart defaults applied consistently
• Please verify LP format generation works correctly

🔍 Test Coverage (src/index.test.ts)

• All 29 tests updated to use new format
• Error validation tests updated for new schema
• Please verify test scenarios cover edge cases adequately

🔍 Documentation (README.md)

• All examples updated to showcase compact format
• Input schema documentation reflects new structure
• Please verify examples are clear and accurate

🔍 Versioning (.changeset/compact-variable-format.md)

• Marked as major breaking change for knope
• Please verify changelog description is appropriate

Testing

✅ All 29 tests pass
✅ TypeScript compilation clean
✅ ESLint passes

This is a breaking change requiring a major version bump due to the incompatible API change.