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.

src/echo.rs

@@ -264,12 +264,16 @@ pub(crate) fn render_drop_index(name: &str) -> String {
 pub(crate) fn render_add_relationship(
     name: &str,
     parent_table: &str,
-    parent_column: &str,
+    parent_columns: &[String],
     child_table: &str,
-    child_column: &str,
+    child_columns: &[String],
     on_delete: ReferentialAction,
     on_update: ReferentialAction,
 ) -> String {
+    // Multi-column FK (ADR-0043): comma-join each side; a
+    // single-column FK is the one-element case.
+    let child_column = child_columns.join(", ");
+    let parent_column = parent_columns.join(", ");
     let mut s = format!(
         "ALTER TABLE {child_table} ADD CONSTRAINT {name} FOREIGN KEY ({child_column}) REFERENCES {parent_table} ({parent_column})"
     );
@@ -325,28 +329,31 @@ pub(crate) fn render_drop_column_cascade(
 pub(crate) fn render_add_relationship_create_fk(
     name: &str,
     parent_table: &str,
-    parent_column: &str,
+    parent_columns: &[String],
     child_table: &str,
-    child_column: &str,
+    child_columns: &[String],
     on_delete: ReferentialAction,
     on_update: ReferentialAction,
-    new_child_column_type: crate::dsl::types::Type,
+    // The child columns `--create-fk` newly creates, with their types
+    // (ADR-0043: one per missing column, typed to the matching parent
+    // PK column's `fk_target_type`). Columns that already existed are
+    // omitted — no `ADD COLUMN` line for them.
+    new_columns: &[(String, crate::dsl::types::Type)],
 ) -> Vec<String> {
-    vec![
-        format!(
-            "ALTER TABLE {child_table} ADD COLUMN {child_column} {}",
-            new_child_column_type.keyword()
-        ),
-        render_add_relationship(
-            name,
-            parent_table,
-            parent_column,
-            child_table,
-            child_column,
-            on_delete,
-            on_update,
-        ),
-    ]
+    let mut lines: Vec<String> = new_columns
+        .iter()
+        .map(|(col, ty)| format!("ALTER TABLE {child_table} ADD COLUMN {col} {}", ty.keyword()))
+        .collect();
+    lines.push(render_add_relationship(
+        name,
+        parent_table,
+        parent_columns,
+        child_table,
+        child_columns,
+        on_delete,
+        on_update,
+    ));
+    lines
 }
 
 /// Append the `NOT NULL` / `UNIQUE` / `DEFAULT` / `CHECK` column-constraint
@@ -953,9 +960,9 @@ mod tests {
         let sql = render_add_relationship(
             "Orders_CustId_to_Customers_id",
             "Customers",
-            "id",
+            &["id".to_string()],
             "Orders",
-            "CustId",
+            &["CustId".to_string()],
             ReferentialAction::NoAction,
             ReferentialAction::NoAction,
         );
@@ -971,9 +978,9 @@ mod tests {
         let sql = render_add_relationship(
             "places",
             "Customers",
-            "id",
+            &["id".to_string()],
             "Orders",
-            "CustId",
+            &["CustId".to_string()],
             ReferentialAction::Cascade,
             ReferentialAction::SetNull,
         );
@@ -1029,14 +1036,14 @@ mod tests {
         let lines = render_add_relationship_create_fk(
             "Customers_id_to_Orders_CustId",
             "Customers",
-            "id",
+            &["id".to_string()],
             "Orders",
-            "CustId",
+            &["CustId".to_string()],
             ReferentialAction::Cascade,
             ReferentialAction::NoAction,
             // Parent PK is `serial` → child FK column is `int`
             // (`Type::fk_target_type` strips auto-gen semantics; ADR-0011).
-            crate::dsl::types::Type::Int,
+            &[("CustId".to_string(), crate::dsl::types::Type::Int)],
         );
         assert_eq!(
             lines.as_slice(),
@@ -1055,12 +1062,12 @@ mod tests {
         let lines = render_add_relationship_create_fk(
             "Items_code_to_Lines_code",
             "Items",
-            "code",
+            &["code".to_string()],
             "Lines",
-            "code",
+            &["code".to_string()],
             ReferentialAction::NoAction,
             ReferentialAction::NoAction,
-            crate::dsl::types::Type::Text,
+            &[("code".to_string(), crate::dsl::types::Type::Text)],
         );
         assert_eq!(lines[0], "ALTER TABLE Lines ADD COLUMN code text");
         // No referential clauses when both default.