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/friendly_enrichment.rs

@@ -420,9 +420,9 @@ fn enrich_fk_insert_resolves_parent_table_column_and_value() {
         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::NoAction,
             ReferentialAction::NoAction,
             false,
@@ -496,9 +496,9 @@ fn enrich_fk_insert_natural_order_multi_value_resolves_via_schema() {
         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::NoAction,
             ReferentialAction::NoAction,
             false,
@@ -570,9 +570,9 @@ fn enrich_fk_delete_resolves_child_table() {
         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::NoAction,
             ReferentialAction::NoAction,
             false,