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/app.rs

@@ -1968,12 +1968,14 @@ impl App {
             ),
             C::AddRelationship {
                 parent_table,
-                parent_column,
+                parent_columns,
                 ..
             } => (
                 Operation::AddRelationship,
                 Some(parent_table.as_str()),
-                Some(parent_column.as_str()),
+                // Single-column facts model (ADR-0019): the first PK
+                // column for a compound FK (ADR-0043).
+                parent_columns.first().map(String::as_str),
             ),
             C::DropRelationship { selector } => match selector {
                 RelationshipSelector::Endpoints {