Implements fixes detailed in docs/NPYITER_FIXES_REQUIRED.md to improve
NumPy compatibility of the NpyIter implementation.

Fix #1: Coalescing Always Runs
- Changed NpyIterRef.Initialize() to always coalesce axes after
  construction unless MULTI_INDEX flag is set
- Matches NumPy's nditer_constr.c line 395-396 behavior

Fix #2: Inner Stride Cache
- Added InnerStrides[MaxOperands] array to NpyIterState
- Added UpdateInnerStrides() method to gather inner strides
- GetInnerStrideArray() now returns contiguous array matching
  NumPy's NpyIter_GetInnerStrideArray() format

Fix #3: op_axes Parameter Implementation
- Added ApplyOpAxes() method to support axis remapping
- Supports -1 entries for broadcast/reduction axes
- Enables reduction operations via custom axis mapping

Fix #4: Multi-Index Support
- Added GetMultiIndex(Span<long>) for coordinate retrieval
- Added GotoMultiIndex(ReadOnlySpan<long>) for coordinate jumping
- Added HasMultiIndex property
- HASMULTIINDEX flag tracked during construction

Fix #5: Ranged Iteration
- Added ResetToIterIndexRange(start, end) for parallel chunking
- Added IterStart, IterEnd, and IsRanged properties
- RANGE flag tracks ranged iteration mode

Fix #6: Buffer Copy Type Dispatch
- Added non-generic CopyToBuffer/CopyFromBuffer overloads
- Runtime dtype dispatch for all 12 NumSharp types
- Enables dtype-agnostic iteration code

Fix #7: Flag Bit Positions Documented
- Added documentation explaining NumSharp's flag bit layout
- Legacy compatibility flags use bits 0-7
- NumPy-equivalent flags use bits 8-15
- Semantic meaning matches NumPy, positions differ

Fix #8: MaxDims Increased to 64
- Changed MaxDims from 32 to 64 to match NPY_MAXDIMS
- Supports high-dimensional array iteration

Test coverage:
- 13 new tests for coalescing, multi-index, ranged iteration,
  inner strides, and MaxDims validation
- All 5666 non-OpenBugs tests pass

Note: Full axis reordering before coalescing (for complete 1D
coalescing of contiguous arrays) not yet implemented. Current
implementation coalesces adjacent compatible axes only.

Explicit the hierarchy — Tier A/B/C were always sub-tiers of Layer 3
(the baked-ufunc layer). Numbering them `3A/3B/3C` makes the
relationship visible at a glance:

  Layer 1  —  ForEach (delegate)
  Layer 2  —  ExecuteGeneric (struct-generic)
  Layer 3  —  ExecuteBinary / Unary / ...  (baked)
  Tier 3A  —  ExecuteRawIL              (sub-tier: custom IL)
  Tier 3B  —  ExecuteElementWise        (sub-tier: templated)
  Tier 3C  —  ExecuteExpression / Call  (sub-tier: DSL)

100 references touched across 6 files:
  docs/website-src/docs/NDIter.md  — prose, TOC, anchor links, worked-
                                    example heading anchors (#6, #7, #8)
  src/NumSharp.Core/Backends/Iterators/NpyExpr.cs       — header comment
  src/NumSharp.Core/Backends/Iterators/NpyIter.Execution.Custom.cs
    — file header, region comments for each tier entry point
  src/NumSharp.Core/Backends/Kernels/ILKernelGenerator.InnerLoop.cs
    — factory method docstrings
  test/NumSharp.UnitTest/Backends/Iterators/NpyIterCustomOpTests.cs
    — class docstring, region comments, 10 test method names
    (TierA_* → Tier3A_*, TierB_* → Tier3B_*, TierC_* → Tier3C_*)
  test/NumSharp.UnitTest/Backends/Iterators/NpyIterCustomOpEdgeCaseTests.cs
    — region comments, 2 test method names (Validate_TierA_* →
    Validate_Tier3A_*)

No behavior changes. 264/264 NpyExpr + custom-op tests pass on net8 +
net10. Full suite still green (0 regressions).