perf(partitioned): bypass gate for idle warm hits

[thomhurst]

Summary

• serve non-expired warm partition hits under the cache lock even when IdleExpiration is configured
• enter the async mutation gate only for creation or when the LRU shows expiration pruning is due
• keep opportunistic expiration and TryGet semantics intact
• add a held-gate regression and permanent warm lookup benchmark

Closes #429

Benchmark

BenchmarkDotNet ShortRun, .NET 10, Windows, i7-12700K:

Case	Mean	Allocated
warm lookup, no expiration	11.87 ns	0 B
warm lookup, 10m idle expiration	31.05 ns	0 B

The idle path now remains a synchronous lock/dictionary/timestamp hit instead of serializing through SemaphoreSlim and sync-over-async.

Validation

• dotnet build Kevlar.slnx -c Release
• dotnet run --project tests/Kevlar.Tests -c Release -f net8.0 --no-build -- --timeout 5m (1,276 passed)
• dotnet run --project tests/Kevlar.Tests -c Release -f net10.0 --no-build -- --timeout 5m (1,310 passed)
• BenchmarkDotNet Dry and Short jobs for PartitioningBenchmarks.Warm_Lookup

[coderabbitai]

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