Feat/lru batch cache eviction

[shaaibu7]

feat(cache): implement LRU batch eviction

Replaces the single-entry oldest-first eviction in cache.ts with a proper LRU strategy that evicts a configurable batch of entries when at capacity.

Changes

• LRU tracking — get and set now refresh a key's position in the Map (delete + re-insert), so the Map's insertion order always reflects LRU → MRU
• Batch eviction — when at capacity, evicts ceil(maxEntries * evictBatchFraction) least-recently-used entries at once (default 10%)
• New config field — evictBatchFraction added to CacheConfig (default 0.1)
• Eviction metrics — getStats() now returns evictions count and logs hit rate on every call

Tests

Added test coverage for:

• LRU ordering after reads and overwrites
• Batch eviction (50% and 10% fractions)
• Cache size never exceeds maxEntries under load (200 rapid inserts)
• Hit rate stays above 80% when recently-set keys are accessed
• evictions field present in PriceCache.getStats()

Closes #41

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