feat: compound-PK foreign-key references — grammar + tests (ADR-0043)

Multi-column FK parsing on both surfaces: DSL from P.(a, b) to
C.(x, y) (parenthesized endpoint; single bare form unchanged) and
SQL FOREIGN KEY (a, b) REFERENCES P(x, y) incl. bare-reference
auto-expand. consume_fk_reference + the table-level/ALTER FK
parsers collect column lists; the from P. completion now offers
( (snapshots updated). 12 integration tests in
tests/it/compound_fk.rs cover parse (both surfaces), engine-enforced
FK, arity + partial-PK + per-pair-type-mismatch refusal,
--create-fk per-column, save->rebuild round-trip, undo (one step),
and single-column preservation. Mark T3 [x]; ADR-0043 implemented.

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__add_relationship__after_child_table_dot_narrows_to_child_columns@after_child_dot.snap

@@ -1,5 +1,6 @@
 ---
 source: tests/typing_surface/add_relationship.rs
+assertion_line: 64
 description: "input=\"add 1:n relationship from Customers.id to Orders.\" cursor=49"
 expression: "& a"
 ---
@@ -25,6 +26,11 @@ Assessment {
                     kind: Identifier,
                     mode: Both,
                 },
+                Candidate {
+                    text: "(",
+                    kind: Punct,
+                    mode: Both,
+                },
             ],
             selected: None,
         },
@@ -52,6 +58,11 @@ Assessment {
                     kind: Identifier,
                     mode: Both,
                 },
+                Candidate {
+                    text: "(",
+                    kind: Punct,
+                    mode: Both,
+                },
             ],
         },
     ),

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__add_relationship__after_parent_table_dot_narrows_to_parent_columns@after_parent_dot.snap

@@ -1,5 +1,6 @@
 ---
 source: tests/typing_surface/add_relationship.rs
+assertion_line: 51
 description: "input=\"add 1:n relationship from Customers.\" cursor=36"
 expression: "& a"
 ---
@@ -20,6 +21,11 @@ Assessment {
                     kind: Identifier,
                     mode: Both,
                 },
+                Candidate {
+                    text: "(",
+                    kind: Punct,
+                    mode: Both,
+                },
             ],
             selected: None,
         },
@@ -42,6 +48,11 @@ Assessment {
                     kind: Identifier,
                     mode: Both,
                 },
+                Candidate {
+                    text: "(",
+                    kind: Punct,
+                    mode: Both,
+                },
             ],
         },
     ),