test: add mutation testing coverage for survived mutants

[ericodx]

Summary

Add 99 mutation-killing tests targeting 76 survived mutants across the codebase. Tests cover boundary conditions, operator replacements, arithmetic mutations, and logical operator swaps in detection, tokenization, pipeline, and reporting stages.

Type of Change

• test: Addition or correction of tests.

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?

• FileDiscovery
• Tokenization
• Normalization
• Detection (Type 1 / Type 2 / Type 3 / Type 4)
• Reporting
• CLI / Configuration
• Cache / Baseline
• None

Testing

• Unit tests added or updated
• Tests use TokenFactory / TempFileHelper / AnalysisHelper where appropriate
• Integration tests added or updated (if detection or pipeline logic changed)
• Both positive cases (expected clones) and negative cases (false positives to avoid) covered
• All tests pass locally (swift test)