feat(graphql): add JS-safe uint64 marshal helpers

Adds MarshalUint64, UnmarshalUint64, and Uint64Marshaler for gqlgen-
compatible uint64 string scalars without a gqlgen dependency.

Co-authored-by: Cursor <cursoragent@cursor.com>

doc.go

@@ -33,6 +33,11 @@
 //
 // [JSONB] provides map-based JSON column scanning for Postgres JSONB and MySQL JSON.
 //
+// # GraphQL scalars
+//
+// [MarshalUint64] and [UnmarshalUint64] provide JS-safe uint64 serialization for
+// gqlgen without importing gqlgen directly.
+//
 // # Pointer and string utilities
 //
 // [Ptr], [Val], [TrimPtr], [StringToFilter], and related helpers for nullable