Reduce matching allocations and skip unrelated ignore rules

[andrew]

Common path matches now avoid heap allocation, rules from unrelated directory scopes are skipped as groups, and directory walks reuse parent checks and discard rules after leaving a subtree. Basename patterns also avoid scanning earlier path components. The public API is unchanged.

Local benchmarks show zero allocations for common matches, about 27% fewer allocated bytes when loading 1,000 scoped rules, and about 14% fewer allocated bytes for a wide directory walk. Timing results remain noisy, with some runs reporting regressions, so these figures describe allocation reductions only.

Adds benchmarks for construction, path depth, rule ordering, sibling scopes, wildcard shapes, parallel matching, and traversal, plus Git conformance coverage for nested walks and tests for deep paths, concurrent matching, and interleaved scopes. Documents how to collect benchmark samples and clarifies parent-directory exclusion.

[Copilot]

🟢 Approval recommended

No unresolved blocking issues were identified.

Pull request overview

Optimizes gitignore matching and directory traversal while preserving the public API.

Changes:

• Groups scoped rules and skips unrelated scopes.
• Reduces matching allocations and reuses parent checks.
• Adds benchmarks, conformance coverage, concurrency tests, and documentation.

File summaries

File	Description
README.md	Documents matching semantics and benchmark methodology.
performance_test.go	Adds performance, concurrency, and traversal tests.
gitignore.go	Implements grouped matching and traversal optimizations.
gitignore_bench_test.go	Adds literal-match benchmarks.
conformance_test.go	Adds nested-walk conformance coverage.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0
• Review effort level: Lite

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