refactor(iterators+docs): cleanup from NpyIter migration

- Delete 4 NPYITER analysis docs (audit, buffered reduce, deep audit,
  numpy differences) — information consolidated into codebase
- Delete 3 NDIterator.Cast files (Complex, Half, SByte) — casting now
  handled by unified NDIterator<T> backed by NpyIter state
- Update NDIterator.cs: minor adjustments from NpyIter backing refactor
- Update ILKernelGenerator.Scan.cs: scan kernel changes
- Update Default.MatMul.Strided.cs: add INumber<T> constraint support
  for generic matmul dispatch preparation
- Update Default.ClipNDArray.cs: initial NpFunc dispatch refactoring
  replacing 6 switch blocks (~84 cases) with generic dispatch methods
- Update np.full_like.cs: minor fix
- Update RELEASE_0.51.0-prerelease.md release notes

Author: Nucs