Fix matrix index slice

[haozhu10015]

This PR fixes #2 by updating matrix indexing/slicing semantics and adding a general matrix selection API.

Changes

• Fix index(&X, i) on matrix expressions to mean X[i, :].
• Fix slice(&X, start, stop) on matrix expressions to mean X[start:stop, :].
• Add AxisIndex and select(&expr, rows, cols) for general 2D selection:
  • row selection
  • column selection
  • scalar element selection
  • rectangular block selection
• Add indexc(&X, j) and slicec(&X, start, stop) aliases for column indexing/slicing.
• Add early bounds validation for invalid scalar/vector/matrix selection.
• Update canonicalization and evaluation so matrix selections use column-major indexing consistently.

Note that in the current implementation, integer indexing will drop the selected axis. For example, index(&X, 1) on a matrix X with shape (3, 4) returns a vector with shape (4,), while slice(&X, 1, 2) preserves the axis and returns a matrix with shape (1, 4).