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/sql_dml_e2e.rs

@@ -318,9 +318,9 @@ fn cascade_fixture(db: &Database, rt: &tokio::runtime::Runtime) {
     rt.block_on(db.add_relationship(
         Some("places".to_string()),
         "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,