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/walking_skeleton.rs

@@ -420,9 +420,9 @@ fn add_relationship_flow_shows_parent_side_with_inbound_section() {
         &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: "CustId".to_string(),
+            child_columns: vec!["CustId".to_string()],
             on_delete: ReferentialAction::Cascade,
             on_update: ReferentialAction::NoAction,
             create_fk: false,
@@ -449,8 +449,8 @@ fn add_relationship_flow_shows_parent_side_with_inbound_section() {
         inbound_relationships: vec![RelationshipEnd {
             name: "Customers_Id_to_Orders_CustId".to_string(),
             other_table: "Orders".to_string(),
-            other_column: "CustId".to_string(),
-            local_column: "Id".to_string(),
+            other_columns: vec!["CustId".to_string()],
+            local_columns: vec!["Id".to_string()],
             on_delete: ReferentialAction::Cascade,
             on_update: ReferentialAction::NoAction,
         }],
@@ -462,9 +462,9 @@ fn add_relationship_flow_shows_parent_side_with_inbound_section() {
         command: 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: "CustId".to_string(),
+            child_columns: vec!["CustId".to_string()],
             on_delete: ReferentialAction::Cascade,
             on_update: ReferentialAction::NoAction,
             create_fk: false,
@@ -504,8 +504,8 @@ fn add_relationship_flow_shows_inbound_section_on_parent() {
         inbound_relationships: vec![RelationshipEnd {
             name: "Customers_Id_to_Orders_CustId".to_string(),
             other_table: "Orders".to_string(),
-            other_column: "CustId".to_string(),
-            local_column: "Id".to_string(),
+            other_columns: vec!["CustId".to_string()],
+            local_columns: vec!["Id".to_string()],
             on_delete: ReferentialAction::Cascade,
             on_update: ReferentialAction::NoAction,
         }],