implement nonsquare matrices

add n_rows and n_cols to matrix, change methods respectively
add additional methods: append_row/column, reserve
assignments should only change upper-left corner of the matrix

Locate the matrix implementation and its existing methods, then trace how dimensions and assignments are currently handled. Define the behavior for n_rows, n_cols, append_row, append_column, reserve, and upper-left assignments, and verify the resulting nonsquare-matrix behavior with the project's available checks.

Written by the indexing model from the issue text.