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/it/main.rs

@@ -9,6 +9,7 @@
 
 mod case_insensitive_names;
 mod column_op_guards;
+mod compound_fk;
 mod engine_vocabulary_audit;
 mod friendly_enrichment;
 mod help_command;