bench: add comparison suite against dashmap and moka

[SF-Zhou]

• Add benches/bench_compare.rs with four groups at 1/4/8 threads: concurrent get (all hits), mixed 90% get / 10% insert, hot-key exclusive entry access (lockmap vs dashmap), and LRU mixed workload under eviction pressure (LruLockMap vs moka)
• Normalize semantics across crates and document fairness notes in the file header (guard-based get copies the value out; moka excluded from the entry group, dashmap from the eviction group)
• Add dashmap 6 and moka 0.12 as dev-dependencies only
• README: add a Benchmarks section with qualitative guidance; no absolute numbers published (to be measured on dedicated hardware)

[Copilot]

Pull request overview

Adds a new Criterion benchmark suite to compare lockmap performance against popular alternatives (dashmap and moka), and documents how to run/interpret these benchmarks.

Changes:

• Introduce benches/bench_compare.rs with multiple multi-threaded workload groups and explicit “fairness notes” to align differing crate semantics.
• Add dashmap and moka as dev-dependencies and register a new bench_compare benchmark target.
• Document benchmark usage and guidance in README.md, and note the addition in CHANGELOG.md.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
README.md	Adds a Benchmarks section with run commands and interpretation guidance.
CHANGELOG.md	Notes the addition of the comparison benchmark suite.
Cargo.toml	Adds dashmap and moka as dev-dependencies and registers the bench_compare bench target.
benches/bench_compare.rs	Implements the new comparison benchmark suite with normalized semantics across crates.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.29%. Comparing base (a022f47) to head (aa13a4b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

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  Hits         2256     2256           
  Misses         16       16           

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