Specialize TreeWalker with struct key comparers

[hadashiA]

Summary

Replaces the central guarded-devirtualization hack (KeyCompare) with value-type generic specialization: TreeWalker is now an abstract non-generic base plus sealed TreeWalker<TComparer> where TComparer : struct, IKeyComparer. Each comparer gets its own runtime instantiation, so every comparison in the B+Tree search loops is a direct, inlinable call — on JIT and on AOT targets (IL2CPP) alike — and SupportsKeyDigest folds to a JIT-time constant, removing the digest branch from the binary search loops.

• IKeyComparer + struct comparers (Int64KeyComparer, AsciiKeyComparer, Uuidv7KeyComparer on .NET 9+, FallbackKeyComparer, DuplicateKeyComparer) in Internal/KeyComparers.cs. Built-in comparers delegate to the sealed encoding singletons, so comparison logic stays in one place.
• LeafNodeReader / InternalNodeReader search methods take TComparer instead of IKeyEncoding.
• All instantiations are spelled out in the TreeWalker.Create factory, so IL2CPP generates them statically. Holders (tables, index queries, iterators) keep referencing the non-generic base — public API is unchanged; virtual dispatch is one call per query instead of one per comparison.
• Custom/plugin encodings (e.g. Ulid) go through FallbackKeyComparer and perform exactly as before. Non-unique secondary indexes get an inlined composite comparer instead of the previous double dispatch.

Benchmark

ReadBenchmark (Release, solo run, Apple M-series / .NET 10), vs current README numbers:

Method	Before	After
DryDB_FindByKey	18.35 us	16.42 us
DryDB_FindByKey_RandomKeys	33.75 us	29.95 us
DryDB_FindByKeyAsync	23.23 us	21.34 us

RandomKeys (-11%) is the most reliable signal since it defeats the branch predictor — consistent with removing per-comparison branches. Parallel variants are within their usual noise. All read paths remain allocation-free; 75/75 tests pass on net8.0/net10.0/netstandard2.1.

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