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/column_op_guards.rs

@@ -132,9 +132,9 @@ fn add_relationship_refuses_internal_tables() {
         .block_on(db.add_relationship(
             None,
             internal.clone(),
-            "name".to_string(),
+            vec!["name".to_string()],
             "C".to_string(),
-            "x".to_string(),
+            vec!["x".to_string()],
             ReferentialAction::NoAction,
             ReferentialAction::NoAction,
             false,
@@ -161,9 +161,9 @@ fn add_relationship_refuses_internal_tables() {
         .block_on(db.add_relationship(
             None,
             "P".to_string(),
-            "id".to_string(),
+            vec!["id".to_string()],
             internal,
-            "x".to_string(),
+            vec!["x".to_string()],
             ReferentialAction::NoAction,
             ReferentialAction::NoAction,
             false,