Improve the IndexStoreFinder architecture and testability

[michaelversus]

🏗️ Improve Architecture & Add Comprehensive Test Coverage

Summary

This MR refactors the IndexStore module with protocol-based abstractions, enabling full testability while maintaining the same runtime behavior. Adds 800+ lines of new tests covering the core symbol search functionality.

Changes

🎯 Architecture Improvements

New File	Purpose
IndexStoreProviding.swift	Protocol abstractions for IndexStore, UnitReader, RecordReader, and SymbolOccurrence
IndexStore+Providing.swift	Conformance of Apple's IndexStore types to the new protocols
RecordIndex.swift	Encapsulates record name → source file path mapping
SymbolQuery.swift	Encapsulates symbol search criteria with matching logic
SymbolKind+Parsing.swift	Extracts SymbolKind parsing into a clean extension
SymbolMatching.swift	Protocol for symbol comparison, enabling mock injection

✅ Test Coverage Added

Test File	Tests
IndexStoreFinderTests.swift	+324 lines – Mock-based tests for parallel search, filtering, edge cases
RecordIndexTests.swift	+297 lines – Building index, source path resolution, deduplication
SymbolQueryTests.swift	+183 lines – Name matching, kind filtering, all symbol types

🔧 Other Changes

• Updated README to highlight parallel scanning capability
• Bumped version: 0.1.5 → 0.2.1
• Introduced ThreadSafeSet for cleaner concurrent result collection
• Added @preconcurrency import IndexStore for Swift 6 compatibility

Why This Matters

• Testability: Core search logic is now fully testable without hitting the filesystem
• Separation of Concerns: Each struct has a single responsibility
• Maintainability: Protocol boundaries make it easy to swap implementations or extend functionality
• Confidence: 800+ lines of tests ensure refactoring didn't break existing behavior

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.40260% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.50%. Comparing base (e84774f) to head (242a716).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...wiftFindRefs/IndexStore/IndexStore+Providing.swift	0.00%	13 Missing ⚠️
...ts/SwiftFindRefs/IndexStore/RecordIndexTests.swift	98.94%	2 Missing ⚠️
...es/SwiftFindRefs/IndexStore/IndexStoreFinder.swift	96.87%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
+ Coverage   87.64%   95.50%   +7.85%     
==========================================
  Files          14       20       +6     
  Lines         502     1089     +587     
==========================================
+ Hits          440     1040     +600     
+ Misses         62       49      -13     

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