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/walker/mod.rs

@@ -3514,9 +3514,9 @@ mod tests {
             Command::AddRelationship {
                 name: None,
                 parent_table: "Customers".to_string(),
-                parent_column: "id".to_string(),
+                parent_columns: vec!["id".to_string()],
                 child_table: "Orders".to_string(),
-                child_column: "customer_id".to_string(),
+                child_columns: vec!["customer_id".to_string()],
                 on_delete: ReferentialAction::default_action(),
                 on_update: ReferentialAction::default_action(),
                 create_fk: false,
@@ -3535,9 +3535,9 @@ mod tests {
             Command::AddRelationship {
                 name: Some("cust_orders".to_string()),
                 parent_table: "Customers".to_string(),
-                parent_column: "id".to_string(),
+                parent_columns: vec!["id".to_string()],
                 child_table: "Orders".to_string(),
-                child_column: "customer_id".to_string(),
+                child_columns: vec!["customer_id".to_string()],
                 on_delete: ReferentialAction::Cascade,
                 on_update: ReferentialAction::SetNull,
                 create_fk: true,