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/output_render.rs

@@ -78,6 +78,16 @@ pub fn render_data_table(data: &DataResult) -> Vec<String> {
 /// — `References:` / `Referenced by:` blocks below as plain
 /// indented text (relationship visualization is its own
 /// future ADR per §5 OOS-1).
+/// Display a relationship-endpoint column list (ADR-0043): the bare
+/// column for a single-column FK, `(a, b)` for a compound one.
+fn cols_disp(cols: &[String]) -> String {
+    if cols.len() == 1 {
+        cols[0].clone()
+    } else {
+        format!("({})", cols.join(", "))
+    }
+}
+
 #[must_use]
 pub fn render_structure(desc: &TableDescription) -> Vec<String> {
     let mut out: Vec<String> = Vec::new();
@@ -112,9 +122,9 @@ pub fn render_structure(desc: &TableDescription) -> Vec<String> {
         for r in &desc.outbound_relationships {
             out.push(format!(
                 "  {} → {}.{}  ({}, on delete {}, on update {})",
-                r.local_column,
+                cols_disp(&r.local_columns),
                 r.other_table,
-                r.other_column,
+                cols_disp(&r.other_columns),
                 r.name,
                 r.on_delete,
                 r.on_update,
@@ -127,8 +137,8 @@ pub fn render_structure(desc: &TableDescription) -> Vec<String> {
             out.push(format!(
                 "  {}.{} → {}  ({}, on delete {}, on update {})",
                 r.other_table,
-                r.other_column,
-                r.local_column,
+                cols_disp(&r.other_columns),
+                cols_disp(&r.local_columns),
                 r.name,
                 r.on_delete,
                 r.on_update,
@@ -769,8 +779,8 @@ mod tests {
             inbound_relationships: vec![RelationshipEnd {
                 name: "cust_orders".to_string(),
                 other_table: "Orders".to_string(),
-                other_column: "cust_id".to_string(),
-                local_column: "id".to_string(),
+                other_columns: vec!["cust_id".to_string()],
+                local_columns: vec!["id".to_string()],
                 on_delete: ReferentialAction::Cascade,
                 on_update: ReferentialAction::NoAction,
             }],