feat: Add C.F.R. part/section separation support

[beshkenadze]

Summary

This PR adds proper support for separating C.F.R. citations into part and section components, eliminating the need for manual string splitting of citations like "778.113" into part "778" and section "113".

Changes Made

Core Implementation

• ✅ Add part field to Metadata interface
• ✅ Add part, sectionOnly, and backward-compatible section getters to FullLawCitation
• ✅ Update C.F.R. regex patterns in laws.json to capture part and section separately
• ✅ Fix ID resolution logic in resolve.ts to handle part.section substitution correctly

Testing

• ✅ Add comprehensive test suite (tests/cfr-part-section.test.ts) with 11 test cases
• ✅ Update existing tests to work with new part/section structure
• ✅ All 423 tests passing with 0 failures

API Changes

New Properties Available

const citation = citations[0] as FullLawCitation

// New functionality
const part = citation.part           // "778" 
const sectionOnly = citation.sectionOnly  // "113"

// Backward compatible (existing code continues to work)
const combined = citation.section    // "778.113"

Before vs After

Before (manual parsing required):

const [part, section] = citation.section.split('.')

After (direct access):

const part = citation.part        // "778"
const section = citation.sectionOnly  // "113"  
const combined = citation.section     // "778.113" (backward compatible)

Backward Compatibility

✅ No breaking changes - All existing code continues to work unchanged.
✅ The section getter still returns the combined "778.113" format for backward compatibility.
✅ New functionality is additive only.

Test Results

✅ 423 tests passing
❌ 0 tests failing  
📊 2,146 assertions executed successfully

Coverage

• Single section parsing: 29 C.F.R. § 778.113 → part: "778", section: "113"
• Multiple sections: 29 C.F.R. §§ 778.113, 778.114 (maintains original behavior)
• Complex sections: 29 C.F.R. § 1910.1200 → part: "1910", section: "1200"
• Pin cites: 29 C.F.R. § 778.113(a) → part: "778", section: "113", pinCite: "(a)"
• ID resolution: Id. § 778.114(a)(5) correctly substitutes both part and section

Demo

The demo script (demo-citation-extraction.ts) shows the new functionality in action with real C.F.R. and U.S.C. citations.

Resolves the user's request to avoid manual string parsing of C.F.R. part.section identifiers.

[beshkenadze]

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