Extract weighted partitioning into WeightedLfuCapacityPartition

[bitfaster]

Summary

Refactor (no behavior change): extract the weighted partitioning logic out of the large
ConcurrentLfuCore into a dedicated WeightedLfuCapacityPartition, mirroring LfuCapacityPartition.

Stacked on #814 (weighted LFU). Base is users/alexpeck/weightedlfu so the diff shows only this
refactor; retarget to main once #814 merges.

What moves

• New internal interface ICapacityPartition { int Capacity { get; } } (Lfu namespace; distinct
  from the existing public Lru.ICapacityPartition). Both LfuCapacityPartition and the new
  WeightedLfuCapacityPartition implement it.
• New WeightedLfuCapacityPartition holds the window/main weighted maximums
  (Maximum/WindowMaximum/MainProtectedMaximum), the hill-climb state, all weighted tuning
  consts (MainPercentage, MainProtectedPercentage, AdmitHashDosThreshold, the HillClimber*
  set, SmallCacheThreshold, QueueTransferThreshold), the maximum-initialization, and the climb
  (OptimizePartitioning + DetermineWeightedAdjustment/IncreaseWindow/DecreaseWindow/CopySign).
• ConcurrentLfuCore now holds an ICapacityPartition and is ~150 lines lighter.

Design

The weighted climb physically moves nodes between the core's LRU lists and mutates the core's
weighted sizes, so it isn't a pure calculator like LfuCapacityPartition. The partition's
OptimizePartitioning<K,V,N,P,E>(ref ConcurrentLfuCore<…> cache, ICacheMetrics metrics, int sampleThreshold) reaches back into the core — the same pattern as the existing
INodePolicy.ExpireEntries(ref ConcurrentLfuCore<…> cache). Conceptual boundary: the maximums
move to the partition; the sizes (weightedSize/windowWeightedSize/mainProtectedWeightedSize)
remain in the core as runtime accounting (the climb mutates them via the ref). The core exposes the
minimum needed members as internal for the reach-back; the count and weighted paths select the
concrete partition via the compile-time IsWeighted flag (cast helpers CountCapacity/WeightedCapacity).

Testing

• Behavior-preserving: full suite green (1579) across the count and weighted paths; builds on all
  TFMs (netstandard2.0, netcoreapp3.1, net6.0, net10.0, net48); dotnet format clean.
• New WeightedLfuCapacityPartitionTests covers the ctor maximum split (mirrors
  LfuCapacityPartitionTests); the climb itself is exercised end-to-end by the existing
  WeightedNodePolicyTests window-adaptation test.

Notes / possible follow-ups

• The reach-back widens the core's internal surface slightly (lists, two size fields,
  ReFitProtectedWeighted). A narrower set of encapsulated move helpers is a possible follow-up.
• AdmitHashDosThreshold is admission tuning (used by AdmitCandidateWeighted) rather than
  partitioning, but moves with the other tuning consts per the requested grouping.

Draft

Opened as a draft for review of the boundary and the ICapacityPartition shape.