MAPCOLS / MAPROWS: support in-place line expansion (fnc returns multiple lines)

[jimmytacks]

Follow-up to #260 / #264.

The v1 MAPCOLS / MAPROWS pair ships with the same-length contract: fnc takes one column (or row) and must return a line of the same length. This keeps the implementation clean and fully testable by reusing the BICOL / BIROW engine directly.

The deferred capability

The original idea (#260) noted an unlock neither competitor wrapper had:

in-place column expansion — splice multiple result columns back at the original position.

i.e. allow fnc to return an m×k block for a selected column, and splice all k result columns back in at the source position (growing the array), rather than requiring k = 1.

Why it was deferred

• Variable per-line widths mean the result is no longer a fixed m×nSel block, so BICOL can't be reused as-is.
• The scatter/merge step needs NA-padding + TOCOL collapse tricks (see the combinatorial-generation note in the create-lambda skill) to assemble a ragged result into a rectangle.
• Harder to test and higher risk for the initial PR.

Acceptance

• MAPCOLS(array, cols, fnc) where fnc returns m×k splices k columns back at each source position (placement 0), shifting later columns right.
• Append / prepend modes stack the (wider) result blocks.
• Symmetric behaviour for MAPROWS (row expansion downward).
• Decide and document behaviour when different selected lines expand by different widths.