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.

tests/it/iteration2_persistence.rs

@@ -192,9 +192,9 @@ fn delete_with_cascade_rewrites_both_csvs() {
         db.add_relationship(
             None,
             "Customers".to_string(),
-            "id".to_string(),
+            vec!["id".to_string()],
             "Orders".to_string(),
-            "CustId".to_string(),
+            vec!["CustId".to_string()],
             ReferentialAction::Cascade,
             ReferentialAction::NoAction,
             false,
@@ -424,9 +424,9 @@ fn project_yaml_carries_relationship_after_add() {
         db.add_relationship(
             None,
             "Customers".to_string(),
-            "id".to_string(),
+            vec!["id".to_string()],
             "Orders".to_string(),
-            "CustId".to_string(),
+            vec!["CustId".to_string()],
             ReferentialAction::Cascade,
             ReferentialAction::NoAction,
             false,