COLUMN MAPPING — multi-line expressions

A follow-up on the per-column width/collapse work. The output-side expression becomes a small code editor: mono font (JetBrains Mono) with SQL syntax colours. Press Shift+Enter or Alt+Enter to add a line — each line adds exactly one tree-row of height (22px), pushing the rows below it down. The handle, name, type code and physical type stay pinned to the first line. Tab inserts a 2-space indent. Plain Enter commits; Esc reverts. Try editing the tier and amount_p3k expressions.