docs: ADR-0043 compound-PK foreign-key references (T3); accepted

Audit found single-column FK woven through ~15-20 sites; earns an
ADR. Decision: reference the parent's full compound PK, matched
positionally to an equal-length child list, per-pair type compat.
DSL `from P.(a,b) to C.(x,y)`; SQL `FOREIGN KEY (x,y) REFERENCES
P(a,b)` with bare-FK auto-expansion. Storage follows the existing
primary_key: [...] list convention (yaml columns: [a,b], uniform
JSON in unchanged metadata TEXT cols); back-compat not required,
so no migration. Also marks T3's verified scope.

docs/requirements.md

@@ -405,10 +405,24 @@ since ADR-0027.)
   (`with pk a(int),b(int)`), **storage** (`primary_key:
   Vec<String>`), and **display** are present and tested.
   **Missing: a FK that *references* a compound PK** —
-  `db.rs:6822-6836` enforces a single `parent_column: String`;
-  a bare `REFERENCES parent` on a compound-PK table is refused as
-  ambiguous, and multi-column FK target syntax is not in the
-  grammar. This is the one open end-to-end leg of T3.)*
+  `db.rs` resolve/alter FK paths enforce a single
+  `parent_column: String`; a bare `REFERENCES parent` on a
+  compound-PK table is refused as ambiguous, and multi-column FK
+  target syntax is not in the grammar. This is the one open
+  end-to-end leg of T3 — but a **codebase audit (2026-06-09)
+  found it is not a small finish**: single-column FK is woven
+  through ~15–20 sites across 6+ files — the
+  `__rdbms_playground_relationships` table schema, the
+  `RelationshipSchema` struct, the **`project.yaml` relationship
+  format** (`RawEndpoint { column }`), both grammar surfaces
+  (`add 1:n relationship` + SQL `FOREIGN KEY`), the executor's FK
+  DDL emission, and the per-column type-compat check. It needs a
+  **migration** (the metadata-table + yaml-format change, F3) and
+  an **ADR** to settle the design forks: compound-PK matching
+  policy (must an FK reference *all* PK columns, or a subset?),
+  per-pair type-compat semantics, the yaml multi-column shape, and
+  back-compat for existing single-column projects. So this leg is
+  ADR-first, not a sweep item.)*
 
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