Performance Testing Documentation

Create docs/performance-testing.md documenting the performance testing framework.

Sections:
1. Overview - What the framework does, goals
2. Running Benchmarks
   - make bench command
   - Running specific benchmarks
   - Interpreting output (ns/op, allocs/op)
3. Profiling and Analysis
   - Viewing CPU profiles with pprof
   - Reading flamegraphs
   - Memory profiling
   - Finding hotspots
4. User Diagnostics
   - bd doctor --perf usage
   - Sharing profiles with bug reports
   - Understanding the report output
5. Comparing Performance
   - Using benchstat for before/after comparisons
   - Detecting regressions
6. Tips for Optimization
   - Common patterns
   - When to profile vs benchmark

Style:
- Concise, practical examples
- Screenshots/examples of pprof output
- Clear command-line examples
- Focus on workflow, not theory