feat: add StringKSUID and BinaryKSUID types

Replace the UUID wrapper with storage-specific KSUID types that share
GraphQL string transit but use distinct SQL Scan/Value encodings.

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

doc.go

@@ -21,8 +21,8 @@
 //
 // # Identifier types
 //
-// [UUID] wraps segmentio/ksuid for GraphQL APIs. [JSONB] provides map-based
-// JSON column scanning for Postgres JSONB and MySQL JSON.
+// [StringKSUID] and [BinaryKSUID] wrap segmentio/ksuid with storage-specific
+// SQL encoding. [JSONB] provides map-based JSON column scanning.
 //
 // # Pointer and string utilities
 //