feat(fk): Foreign Key Enforcement — ON DELETE/UPDATE CASCADE, SET NULL, RESTRICT

- Add tkRestrict token to lexer
- Parse ON DELETE CASCADE/SET NULL/RESTRICT and ON UPDATE CASCADE/SET NULL/RESTRICT
  in both table-level and column-level FK constraints
- Add fkOnDelete/fkOnUpdate to ColumnDef and ForeignKeyDef
- Fix table-level FK constraint application (third pass after columns are created)
- Implement enforceFkOnDelete, enforceFkOnUpdate, enforceFkOnChildUpdate helpers
- Wire FK enforcement into DELETE and UPDATE execution paths
- Add 9 regression tests covering all FK actions

src/barabadb/query/lexer.nim

@@ -85,6 +85,7 @@ type
     tkForeign
     tkReferences
     tkCascade
+    tkRestrict
     tkUnique
     tkCheck
     tkDefault
@@ -308,6 +309,7 @@ const keywords*: Table[string, TokenKind] = {
   "foreign": tkForeign,
   "references": tkReferences,
   "cascade": tkCascade,
+  "restrict": tkRestrict,
   "unique": tkUnique,
   "check": tkCheck,
   "default": tkDefault,