Refactor cache API: remove value handling

[bootjp]

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[Copilot]

Pull request overview

This PR refactors the AdaptSize cache library from a full key-value store to a policy-only admission controller. The cache no longer stores actual values or manages LRU eviction—instead, callers integrate it with their own storage layer and use the Request API to record accesses and get admission decisions for cache misses. The core AdaptSize algorithm (probabilistic admission via exp(-size/c) with automatic c-parameter tuning) remains unchanged.

Key Changes:

• Replaced Get/Set methods with a single Request method that records metrics and returns admission decisions
• Removed internal storage structures (items map, lru list, entry type) and capacity tracking
• Updated all tests to use the new API pattern where callers specify Hit status

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
go.mod	Updates Go version (contains invalid version number)
adaptsize/adaptsize.go	Removes value storage and LRU eviction; adds Request API for admission decisions only
adaptsize/adaptsize_test.go	Updates existing tests to Request API; adds new tests for hit recording, oversize handling, EMA computation, and no-data tuning behavior
README.md	Updates usage example to demonstrate Request API pattern with caller-managed cache storage

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