make even more adaptive (#4)

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## Summary by CodeRabbit

## Release Notes

* **New Features**
  * Self-tuning frequency-based eviction with adaptive learning signals
  * Ghost entries for improved scan resistance
  * Capacity-driven configuration approach

* **API Changes**
* New `AverageLearnedThresholds()` method to retrieve adaptive
thresholds
* Introduced `ConfigFromCapacity()` and `ConfigFromMemorySize()` factory
methods
* Changed `AdaptiveStats.K` field type; added `LearnedRateLow`,
`LearnedRateHigh`, and `WindowHitRate` fields
  * Removed legacy hardware detection methods

* **Documentation**
* Updated README with self-tuning eviction terminology and configuration
examples

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Signed-off-by: Robert Landers <landers.robert@gmail.com>

Author: withinboredom