build: Add generated code performance benchmarks

[DrBarnabus]

Summary

• Add CompositeKey.Benchmarks console project (net8.0/net9.0/net10.0) with BenchmarkDotNet 0.15.8
• Define five representative key types covering all major code-generation paths: exact-length string.Create, interpolation formatting, DefaultInterpolatedStringHandler with repeating sections, fast-path enum helpers, partition/sort splitting, same-separator repeating, ImmutableArray collection construction, and SpanParsable item parsing
• Benchmark ToString, Parse, TryParse, ToPartitionKeyString, and ToSortKeyString with [MemoryDiagnoser] tracking throughput, allocations, and GC collections
• Add BenchmarkDotNet.Artifacts/ to .gitignore, benchmarks README, and CLAUDE.md section

Test plan

• Solution builds with zero warnings and zero errors
• All existing tests pass (2,830 across 4 test projects × 3 TFMs)
• All 17 benchmarks execute successfully via dotnet run -c Release --project src/CompositeKey.Benchmarks -- --filter "*" --job short

Closes #40

Summary by CodeRabbit

Release Notes

• New Features

  • Added comprehensive benchmarking infrastructure including performance measurement suites for multiple composite key types.

• Documentation

  • Added benchmark documentation with usage instructions and running guidelines.

• Chores

  • Updated project configuration files and source control ignore rules to support the benchmarking framework.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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Walkthrough

This pull request introduces a new BenchmarkDotNet-based benchmarking project for CompositeKey, establishing performance baselines across representative key types targeting net8.0, net9.0, and net10.0, with benchmarks for ToString, Parse, and TryParse operations.

Changes

Cohort / File(s)	Summary
Project Configuration .gitignore, CompositeKey.slnx, src/CompositeKey.Benchmarks/CompositeKey.Benchmarks.csproj, src/Directory.Packages.props	Added BenchmarkDotNet artifacts to ignore patterns, registered the new benchmarks project in the solution, configured the benchmark project to target multiple frameworks with BenchmarkDotNet package dependency, and added BenchmarkDotNet 0.15.8 to centralised package versions.
Benchmark Implementations src/CompositeKey.Benchmarks/FastPathEnumKeyBenchmarks.cs, src/CompositeKey.Benchmarks/GuidPrimaryKeyBenchmarks.cs, src/CompositeKey.Benchmarks/MixedCompositeKeyBenchmarks.cs, src/CompositeKey.Benchmarks/RepeatingCollectionKeyBenchmarks.cs, src/CompositeKey.Benchmarks/RepeatingSpanParsableBenchmarks.cs	Defined five representative composite key types and corresponding benchmark suites, exercising distinct code-generation paths including simple Guid keys, mixed-type keys with partition/sort separation, repeating collection keys, and fast-path enum formatting, each with ToString, Parse, and TryParse benchmarks.
Project Infrastructure src/CompositeKey.Benchmarks/Program.cs, src/CompositeKey.Benchmarks/packages.lock.json	Added benchmark entry point invoking BenchmarkSwitcher across the assembly, and generated NuGet lockfile specifying dependencies across net8.0, net9.0, and net10.0 frameworks.
Documentation src/CompositeKey.Benchmarks/README.md, CLAUDE.md	Added benchmark project README detailing execution instructions, filtering, multi-framework targeting, and result interpretation, plus documentation entry in CLAUDE.md.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• build: Update to new slnx solution format #23 — Both PRs modify CompositeKey.slnx; this PR adds the benchmarks project to the solution structure.

Suggested labels

enhancement

Poem

🐰 Hop, hop—benchmarks bound,
Stack-alloc'd splits, no heaps around,
Guid and enum paths we trace,
Performance baselines set in place!
Before refactor's gentle paw,
We measure twice to find no flaw. ✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: adding performance benchmarks for generated code to the build system.
Linked Issues check	✅ Passed	All coding requirements from issue #40 are met: benchmarks project created targeting net8.0/9.0/10.0, BenchmarkDotNet added to Directory.Packages.props, five representative key types defined covering all major generation paths, ToString/Parse/TryParse/partition/sort methods benchmarked with MemoryDiagnoser, README provided, and .gitignore updated.
Out of Scope Changes check	✅ Passed	All changes directly support the benchmarks implementation: .gitignore addition, CLAUDE.md documentation, solution file updates, benchmark project structure, representative key types, BenchmarkDotNet package configuration, and supporting files. No unrelated modifications detected.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch build/add-generated-code-benchmarks

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.66%. Comparing base (d922876) to head (f8ddd13).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

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  Coverage   87.66%   87.66%           
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  Files          35       35           
  Lines        2099     2099           
  Branches      344      344           
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  Hits         1840     1840           
  Misses        157      157           
  Partials      102      102           

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