lru: atomic refcount, unlock release path

[nclack]

What

Make the per-entry refcount atomic and drop the cache mutex from the release path. Acquire still takes the mutex — incrementing from zero has to be serialized against the eviction scan, which reads refcounts to decide what's safe to evict.

Why

The refcount is a per-entry counter. It was under the cache mutex only because the field lived inside the entry struct, not because the value needed that lock.

Results

Not a lot of measured performance change on my laptop. The change is still worth landing for cleaner refcount semantics and for the new contention test that exercises the release path under concurrency, but don't expect this workload to get faster.

Key file

src/util/lru.c — refcount, acquire/release, eviction reads.

Closes #108.

[codecov]

Codecov Report

❌ Patch coverage is 67.96117% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.77%. Comparing base (da828e1) to head (cfafcf0).

Files with missing lines	Patch %	Lines
src/threadpool/threadpool.c	0.00%	13 Missing ⚠️
src/util/lru.c	55.55%	7 Missing and 5 partials ⚠️
src/zarr/zarr_meta_cache.c	82.60%	4 Missing ⚠️
src/util/pool.c	81.81%	2 Missing ⚠️
src/store/store_fs.c	93.33%	1 Missing ⚠️
src/zarr/zarr_shard_cache.c	92.85%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (67.96%) is below the target coverage (70.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #111      +/-   ##
==========================================
+ Coverage   55.56%   55.77%   +0.21%     
==========================================
  Files          49       49              
  Lines        6875     6890      +15     
  Branches     1227     1232       +5     
==========================================
+ Hits         3820     3843      +23     
- Misses       2570     2572       +2     
+ Partials      485      475      -10     

Flag	Coverage Δ
unittests	55.77% <67.96%> (+0.21%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/store/store_fs.c	74.09% <93.33%> (+0.24%)	⬆️
src/zarr/zarr_shard_cache.c	64.67% <92.85%> (-1.01%)	⬇️
src/util/pool.c	91.02% <81.81%> (-2.31%)	⬇️
src/zarr/zarr_meta_cache.c	71.03% <82.60%> (-0.69%)	⬇️
src/util/lru.c	79.31% <55.55%> (-1.92%)	⬇️
src/threadpool/threadpool.c	0.00% <0.00%> (ø)

... and 10 files with indirect coverage changes

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