CSV join and merge planner

This tool is a planner and a merge, not a database. Both files are read in this tab and compared key to key after your trim and case options. The forecast numbers are exact row counts, not estimates: a shared key with three rows in A and two in B contributes six rows, empty key cells match nothing the way SQL treats NULL, and the merged CSV can be larger than both inputs when keys repeat. The output keeps every column of file A, appends the non-key columns of file B with a (B) suffix on name clashes, backfills B's key into A's key column for B-only rows, and emits matched rows first in A order. It does no fuzzy matching, no deduplication, and no type casting: cells move over as text, exactly as pasted. Both files need a header row. Nothing is uploaded.