feat: add persistence benchmarks for JSON vs bitcode serialization

[deven96]

Summary

Adds comprehensive persistence benchmarks comparing JSON (serde_json) vs bitcode serialization performance for ahnlich store persistence.

Changes

Benchmarks

• Added new persistence benchmark suite testing 32 scenarios:
  • Sizes: 100, 1K, 10K, 100K entries
  • Index configs: no-index, HNSW (ef_construction=400, m=64)
  • Formats: JSON (serde_json) vs bitcode (with serde feature)
  • Operations: serialize & deserialize
• Realistic test data: 1024-dim embeddings with 3 metadata fields
• File I/O matching real persistence (atomic writes with temp files)

Dependencies

• Removed unused bincode dependency
• Added bitcode (with serde feature) as dev-dependency
• Added chrono for timestamp generation in benchmarks

CI

• Added separate persistence-benchmark.yml workflow
• Triggers only when PR has benchmark-persistence label
• Uses same structure as existing database benchmarks
• Posts comparison results as PR comment

Testing

Benchmarks compile and run successfully:

cargo bench --bench persistence

[github-actions]

Test Results

175 tests   174 ✅  10m 9s ⏱️
 32 suites    0 💤
  4 files      1 ❌

For more details on these failures, see this check.

Results for commit ffc1755.

♻️ This comment has been updated with latest results.