feat: expose composite foreign keys and m2m columns in OpenAPI

Replace the single-column `<fk table='...' column='...'/>` marker with a
unified, composite-capable format `<fk table='...' columns='local:foreign,...'/>`
and emit full column lists for many-to-many markers, so clients can
reconstruct composite relationships.

- Match every real table relationship a column participates in, instead of
  only single-column foreign keys, still preferring tables over the
  view-derived relationships.
- Emit the full ordered local->foreign column mapping in the `<fk>` marker
  and the full source/target column lists in the `<m2m>` marker.
- Add composite foreign key and composite m2m fixtures plus OpenAPI spec
  assertions.

test/spec/fixtures/schema.sql

@@ -1246,6 +1246,58 @@ create table test.being_part (
   primary key(being, part)
 );
 
+-- Tables for testing composite foreign keys in the OpenAPI output
+create table test.comp_product (
+  id int primary key,
+  name text
+);
+
+create table test.comp_component (
+  product_id int not null references test.comp_product(id),
+  component_id int not null references test.comp_product(id),
+  primary key(product_id, component_id)
+);
+
+create table test.comp_product_instance (
+  id int primary key,
+  product_id int not null references test.comp_product(id),
+  unique(id, product_id)
+);
+
+create table test.comp_component_instance (
+  product_instance_id int not null,
+  product_id int not null,
+  component_instance_id int not null,
+  component_id int not null,
+  primary key(product_instance_id, component_id),
+  constraint comp_component_instance_parent_fkey foreign key (product_instance_id, product_id) references test.comp_product_instance(id, product_id),
+  constraint comp_component_instance_child_fkey foreign key (component_instance_id, component_id) references test.comp_product_instance(id, product_id),
+  constraint comp_component_instance_component_fkey foreign key (product_id, component_id) references test.comp_component(product_id, component_id)
+);
+
+-- Tables for testing composite m2m relationships in the OpenAPI output
+create table test.comp_being (
+  a int not null,
+  b int not null,
+  primary key(a, b)
+);
+
+create table test.comp_thing (
+  x int not null,
+  y int not null,
+  primary key(x, y)
+);
+
+create table test.comp_being_thing (
+  a int not null,
+  b int not null,
+  x int not null,
+  y int not null,
+  primary key(a, b, x, y),
+  constraint comp_being_thing_being_fkey foreign key (a, b) references test.comp_being(a, b),
+  constraint comp_being_thing_thing_fkey foreign key (x, y) references test.comp_thing(x, y)
+);
+
 create function test.single_out_param(num int, OUT num_plus_one int) AS $$
   select num + 1;
 $$ language sql;