Complete matrix.mac migration to maximasrc

[adamant-pwn]

Move the rest of matrix.mac into maximasrc: matrix operations, the remaining predicates, and equation-display functions. They become available in core STACK, with co-located tests and generated documentation. Existing matrix.mac includes retain their c/r declaration.

Also fixes existing bugs exposed by the migration:

• Orthogonality checks overwrite M and checkmat; localising them also makes orth_matrixp reject non-matrix lists as documented.
• Equation-display functions overwrite eqns and helper-function definitions.
• mat_disp_eqns fails for a single equation because Maxima collapses its intermediate 1×1 matrix to a scalar.

Correct stale TeX test expectations and document the existing limits on empty matrices.

Stacked on #1906, targeting iss1905. This completes matrix.mac; other contributed libraries and the proposed row_vector/column_vector API remain separate.

Closes #1908.

[sangwinc]

Thanks @adamant-pwn for all your work rearranging and improving this!