Fix disk cache cost tracking and purge edge cases

[naftaly]

Summary

• Fix disk cache not subtracting old file size when overwriting, causing cost to accumulate incorrectly
• Fix URL resource value caching returning stale file sizes after writes
• Fix items being lost from cost-based purge consideration during staleness purge
• Fix potential cost drift when file size read fails but delete succeeds
• Unify purge threshold behavior between memory and disk caches

Test plan

• Added testOverwriteExistingKeyOnDisk
• Added testOverwriteExistingKeyOnCombinedCache
• Added testMultipleOverwritesOnDisk
• All existing tests pass

[github-actions]

🚀 Arsenal Cache Performance Benchmarks

Multi-layer caching with LRU eviction and disk persistence

Test Hardware: Apple M1 (Virtual), 7 GB RAM

Memory Cache Operations

Operation	Items	Time	Per-Op	Status
Get	1000	146.0ms	146.0 μs	⚠️ OK
Set	1000	143.0ms	143.0 μs	⚠️ OK
Set (with purge)	500	251.0ms	502.0 μs	⚠️ OK

Disk Cache Operations

Operation	Items	Time	Per-Op	Status
Get	100	110.0ms	1.10 ms	✅ Excellent
Set	100	139.0ms	1.39 ms	✅ Excellent

Combined Cache (Memory + Disk)

Operation	Items	Time	Per-Op	Status
Get (disk→memory promotion)	100	136.0ms	1.36 ms	✅ Excellent
Set (both layers)	100	884.0ms	8.84 ms	✅ Good

Large Items (1 MB each)

Storage	Items	Time	Per-Item	Status
Disk	20	150ms	7.5ms	✅ Excellent
Memory	50	124ms	2.5ms	✅ Excellent

Throughput (Mixed Read/Write)

Operation	Ops	Time	Ops/sec	Status
Memory (33% write, 67% read)	5,000	129ms	38,760	⚠️ OK

Clear Operations

Storage	Items	Time	Status
Disk	100	142ms	✅ Good
Memory	1000	121ms	✅ Good

Performance Characteristics

Status Legend

• ✅ Excellent/Good: Optimal performance
• ⚠️ OK: Acceptable, monitor in production
• ❌ Review: May need optimization

Architecture

• Memory cache: LRU eviction with cost-based limits
• Disk cache: File-based with staleness and cost eviction
• Combined: Automatic disk→memory promotion on read
• Thread safety: @globalActor isolation via ArsenalActor

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Total benchmarks: 12 passed | Generated 2025-11-30 01:10:23