test: strengthen mutation testing coverage across all modules

[ericodx]

Summary

• Add mutation testing configuration (swift-mutation-testing.yml)
• Strengthen test coverage across 24 existing test files to kill survived mutants
• Add 10 new test files targeting boundary conditions, edge cases, and extracted test suites
• Fix SwiftLint and Codespell violations in test files

Key areas targeted from mutation report analysis:

• CloneDetector: overlap distance boundaries, hash count threshold, subsumption logic, lineCount arithmetic
• Type4Detector: pre-filter ratio boundaries, similarity threshold, combined 60/40 weighting
• AnalysisPipeline: multi-criteria sorting (type → file → startLine), tokenizer path branching
• TextReporter: fragment output lines, clone block separation
• AnalysisResult: sorting comparator edge cases for equal types, token counts, files, and start lines
• SemanticNormalizer, GreedyStringTiler, RollingHash, Type3Detector: relational operator boundaries
• CTokenizer, UnifiedTokenMapper, SuppressionScanner, BaselineStore: boundary and edge case coverage

Type of Change

• test: Addition or correction of tests.
• chore: Other changes that do not fit into the previous categories.

Invariants Checklist

• Detection is deterministic (same input → same output)
• Source files are never modified (tool is read-only)
• Source locations are accurate (file, line, column)
• No false negatives introduced (previously detected clones are still detected)
• Performance not degraded for large codebases
• Swift 6 Strict Concurrency compatible
• Pipeline stages remain stateless pure transformations

Pipeline Impact

Which stages are affected?

• None

Testing

• Unit tests added or updated
• Tests use TokenFactory / TempFileHelper / AnalysisHelper where appropriate
• Both positive cases (expected clones) and negative cases (false positives to avoid) covered
• All tests pass locally (swift test) — 715 tests in 61 suites