- Migrate all .csproj files to SDK-style targeting net10.0
- Add Blake2SCore, Blake2SHasher, Blake2S (sequential BLAKE2s)
- Add Blake2SpHasher, Blake2Sp (parallel BLAKE2s, 8 leaves)
- Add Blake2SConfig, Blake2STreeConfig
- Add ConfigS/ConfigSSetNode to Blake2IvBuilder
- Add KAT-based tests for Blake2S and Blake2Sp (256 unkeyed + 256 keyed each)
- Add Blake2SpHash console app (file hasher, matches C reference output)
- Add csharp/.gitignore for bin/obj build output

- Add ReadOnlySpan<byte> overloads throughout the input path:
  Blake2SCore.HashCore, Blake2SHasher.Update, Blake2SpHasher.Update,
  Blake2S.ComputeHash, Blake2Sp.ComputeHash, Hasher.Update (virtual)
- byte[] overloads retained for compatibility; delegate to span versions
- Blake2SCore-Compress: use MemoryMarshal.Cast on little-endian instead
  of Buffer.BlockCopy; span indexing on big-endian
- Blake2SCore.HashFinal: add Span<byte> output overload as primary;
  LE fast path uses MemoryMarshal.Cast for output; padding uses AsSpan().Clear()
- Blake2SpHasher.Finish: stackalloc leaf hash buffers (eliminates 8 x 32B
  heap allocations per BLAKE2sp call)
- Blake2SHasher.Finish: stackalloc intermediate buffer for truncated output