refactor: relationship model to column lists for compound FK (ADR-0043)

Move the FK column fields String->Vec<String> through all six
layers (AddRelationship/SqlForeignKey AST, RelationshipSchema,
metadata, project.yaml, ReadForeignKey, RelationshipEnd). Metadata
stores comma-joined lists in the existing TEXT cells; project.yaml
endpoints now columns: [a, b] (house style). Executor logic is
multi-column ready: resolve_fk_parent_columns (full-PK F-A +
auto-expand F-D), per-pair type-compat, schema_to_ddl multi-column
emission, pragma FK read grouped by id, auto-name + --create-fk
per-column, multi-column teaching echo. Single-column behaviour
preserved (one-element vecs); all 2181 tests green. The grammar to
parse multi-column input lands next.

src/dsl/parser.rs

@@ -937,9 +937,9 @@ mod tests {
         Command::AddRelationship {
             name: name.map(String::from),
             parent_table: parent.0.to_string(),
-            parent_column: parent.1.to_string(),
+            parent_columns: vec![parent.1.to_string()],
             child_table: child.0.to_string(),
-            child_column: child.1.to_string(),
+            child_columns: vec![child.1.to_string()],
             on_delete,
             on_update,
             create_fk,