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_drop_table.rs

@@ -104,9 +104,9 @@ fn dropping_a_referenced_parent_is_refused() {
         vec![],
         vec![SqlForeignKey {
             name: None,
-            child_column: "pid".to_string(),
+            child_columns: vec!["pid".to_string()],
             parent_table: "parent".to_string(),
-            parent_column: Some("id".to_string()),
+            parent_columns: Some(vec!["id".to_string()]),
             on_delete: rdbms_playground::dsl::ReferentialAction::NoAction,
             on_update: rdbms_playground::dsl::ReferentialAction::NoAction,
         }],