docs: remove or rewrite PERFORMANCE.md (fabricated data)

[galligan]

Summary

docs/architecture/PERFORMANCE.md contains fabricated benchmark data, non-existent APIs, and claims about unimplemented features.

Findings

Issue ID	Description
PERF-001	Contains fabricated benchmarking data with no backing tests
PERF-002	Documents non-existent public APIs (SearchEngine::with_cache_config, etc.)
PERF-003	Claims unimplemented features (LRU cache, memory mapping)

Recommendation

Either:

1. Delete the file entirely if performance documentation isn't needed
2. Rewrite with actual benchmarks from cargo bench results

Files to Update

• docs/architecture/PERFORMANCE.md - Delete or rewrite

Source of Truth

• benches/ directory for actual benchmark code
• cargo bench output for real numbers

Priority

Critical - Misleading users about performance characteristics.

[linear]

BLZ-313