perf: use HashSet for file exclusion, increase test coverage to 93%

[7a6163]

• Change collect_excluded_files to return HashSet for O(1) exclusion lookups instead of O(n) Vec::contains
• Use saturating_sub in print_fix_statistics to prevent potential arithmetic underflow
• Add 216 new tests (184 unit + 6 integration + 26 existing) covering all modules: offense, output, file_traverser, config, comment_directives, analyzer, fix, ast_helpers, and all scanners
• Add module_eval heredoc fixture test
• Add CLI integration tests for --fix, --format, and config loading
• Coverage: 61.87% → 93.10% (1106/1188 lines)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.74059% with 27 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/ast_helpers.rs	94.52%	25 Missing ⚠️
src/analyzer.rs	98.55%	1 Missing ⚠️
src/scanner/for_loop_scanner.rs	96.15%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/comment_directives.rs	99.20% <100.00%> (+6.67%)	⬆️
src/config.rs	100.00% <100.00%> (+11.47%)	⬆️
src/file_traverser.rs	98.47% <100.00%> (+22.71%)	⬆️
src/fix.rs	100.00% <100.00%> (+16.83%)	⬆️
src/offense.rs	100.00% <100.00%> (+31.11%)	⬆️
src/output.rs	99.01% <100.00%> (+77.09%)	⬆️
src/scanner/method_call_scanner.rs	97.00% <100.00%> (+3.23%)	⬆️
src/scanner/method_definition_scanner.rs	97.53% <100.00%> (+11.40%)	⬆️
src/scanner/rescue_scanner.rs	96.77% <100.00%> (+0.94%)	⬆️
src/analyzer.rs	96.11% <98.55%> (+15.93%)	⬆️
... and 2 more

... and 1 file with indirect coverage changes

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