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/persistence/mod.rs

@@ -245,9 +245,13 @@ pub struct IndexSchema {
 pub struct RelationshipSchema {
     pub name: String,
     pub parent_table: String,
-    pub parent_column: String,
+    /// Parent PK column(s); one element for single-column, ordered
+    /// list for a compound-PK FK (ADR-0043). Paired positionally
+    /// with `child_columns`.
+    pub parent_columns: Vec<String>,
     pub child_table: String,
-    pub child_column: String,
+    /// Child column(s), positionally paired with `parent_columns`.
+    pub child_columns: Vec<String>,
     pub on_delete: ReferentialAction,
     pub on_update: ReferentialAction,
 }