Port v3 RaggedEnd for per-column end in ragged arrays - #555

Implements the v3 RaggedEnd/RaggedRange mechanism in the ragged sublibrary
so that `r[end, i]` returns the last element of column `i` (not the global
max). This matches the original v3 VectorOfArray ragged behavior exactly:

  r = RaggedVectorOfArray([[1.0, 2.0], [3.0, 4.0, 5.0]])
  r[end, 1]    # 2.0 (last of column 1)
  r[end, 2]    # 5.0 (last of column 2)
  r[1:end, 1]  # [1.0, 2.0]
  r[1:end, 2]  # [3.0, 4.0, 5.0]

`lastindex(r, d)` returns a `RaggedEnd` sentinel for leading dimensions,
which is resolved per-column at indexing time via `_resolve_ragged`.

Addresses feedback from @JoshuaLampert.

Co-Authored-By: Chris Rackauckas <[email protected]>
Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Replace the partial reimplementation with the COMPLETE v3 vector_of_array.jl
code, mechanically transformed:
- VectorOfArray → RaggedVectorOfArray
- DiffEqArray → RaggedDiffEqArray
- AbstractVectorOfArray → AbstractRaggedVectorOfArray
- AbstractDiffEqArray → AbstractRaggedDiffEqArray

This preserves ALL v3 behavior exactly:
- A[i] returns the i-th inner array (not scalar)
- A[:, i] returns inner array without zero-padding
- RaggedEnd for per-column `end` resolution
- Iteration over inner arrays
- Full broadcasting, copy, zero, similar, fill!, push!, etc.

Tests are the complete v3 test suite (basic_indexing + interface_tests)
transformed with same type renames, plus new tests for interp/dense,
conversion, SymbolicIndexingInterface, and type hierarchy.

430 tests pass.

Co-Authored-By: Chris Rackauckas <[email protected]>
Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>