Decomposes the v1 concordance.csv into the component tables described
in PLAN.md, without changing any value (the baseline, PLAN.md 7.5):
forms.csv,parts.csv,tables.csv: one row per form, part, table.variables.csv: one row per variable. Variable-level columns hold the most common value across the variable's xpaths (ties go to the first xpath in v1 order).xpaths.csv: one row per xpath with the xpath-level columns and its v1 row number (v1_order).xpath_overrides.csv: every place where an xpath's v1 value differs from its variable's value. These are the v1 conflicts; nothing is lost, and resolving one means deleting its row (with a change-log entry).families.csv: empty; families with alternates are defined in Phase 4.
Usage
split_v1(v1_path = v1_path_default(), out_dir = NULL)