perf: split admission and eviction locks

[huynhanx03]

Summary

Split the policy lock into independent admission and eviction locks. This prevents eviction work in Add from blocking TinyLFU frequency updates in processItems and allows eviction metadata reads to run concurrently.

Changes

• Protect TinyLFU with admitMu and sampledLFU with evictMu.
• Release the eviction lock before frequency estimation and validate sampled victims before deletion.
• Add concurrency tests and focused policy benchmarks.

Benchmark

CPU: Apple M5 (darwin / arm64)

Name	Before	After	Change
ProcessItems	301.1 ns/op	311.4 ns/op	+3.4%
ProcessItemsDuringEviction	47,031 ns/op	431.5 ns/op	-99.1%
Frequency throughput during eviction	85,050 keys/s	9,270,112 keys/s	109x
Concurrent Has	164.6 ns/op	122.9 ns/op	-25.3%
Concurrent Cost	145.2 ns/op	103.4 ns/op	-28.8%
Concurrent Cap	102.1 ns/op	77.43 ns/op	-24.2%
Add with eviction	908.1 ns/op	1,079 ns/op	+18.8%

Checklist

• Code compiles correctly and linting passes locally
• Tests added for new functionality, or regression tests for bug fixes added as applicable

[huynhanx03]

Hi @mangalaman93 , could you review, thanks.

[huynhanx03]

Hi @matthewmcneely, help me pls.