Add SimilarityLFU cache admission policy - #101
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Introduces AdmissionPolicy, a new decision point separate from EvictionPolicy: instead of deciding what to remove once the cache is full, it decides whether a cache-miss item is worth caching at all. Wired into VCacheConfig/Cache as fully opt-in (defaults to AlwaysAdmitPolicy, preserving existing behavior). SimilarityLFUAdmissionPolicy adapts TinyLFU's "don't admit until it recurs" idea to vCache's semantic cache: since two paraphrased queries hash completely differently, exact-key frequency sketches don't apply here, so recurrence is checked via embedding cosine-similarity against a small bounded probation list instead. Benchmarked against a real 1591-row workload across 6 eviction policies and 7 cache sizes: it substantially improves hit ratio and precision under tight cache pressure (e.g. avg hit ratio 2.13% -> 5.82% at 40MB), but consistently hurts once the cache is generous enough that little is actually being evicted (e.g. 13.26% -> 8.10% at 100MB). Documented as a tight-cache-pressure tool, not a universal win.
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Introduces AdmissionPolicy, a new decision point separate from EvictionPolicy: instead of deciding what to remove once the cache is full, it decides whether a cache-miss item is worth caching at all. Wired into VCacheConfig/Cache as fully opt-in (defaults to AlwaysAdmitPolicy, preserving existing behavior).
SimilarityLFUAdmissionPolicy adapts TinyLFU's "don't admit until it recurs" idea to vCache's semantic cache: since two paraphrased queries hash completely differently, exact-key frequency sketches don't apply here, so recurrence is checked via embedding cosine-similarity against a small bounded probation list instead.