feat: blast-radius fitness component (#151)

[yxlyx]

Summary

• parseDiffFiles(): Extracts unique file paths from diff --git lines
• parseDiffSymbols(): Extracts symbol names from hunk headers (@@) and changed lines (+/-)
• computeBlastScore(): Pure computation: 1.0 - (unrelated_blast / total_blast_files), clamped [0,1]
• blastRadiusScore(): Full pipeline: parse diff -> search refs via search.zig -> compute score
• Also fixes const/var bug in search.zig searchRefs tests (const ArrayList can't call mutable deinit)

Score semantics:

• 1.0 = no external references, or all blast files are in the touched set
• 0.0 = all blast files are outside the touched set (maximum sprawl)
• 0.5 = half of referencing files were not modified by the diff

Test plan

• zig test src/evolver.zig — 51/51 tests pass (27 evolver + 24 search)
• parseDiffFiles: extracts unique paths, deduplicates, handles empty
• parseDiffSymbols: extracts from hunk headers and changed lines, handles empty
• computeBlastScore: empty blast -> 1.0, all touched -> 1.0, none touched -> 0.0, partial -> 0.5
• Single file with one unrelated reference -> 0.5

Closes #151

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