Write every builtin one way

A builtin could be spelled as prep (args parsed once, at compile) or call
(args parsed per render), reaching the same closure by two routes. Nothing
enforced the "exactly one of" invariant: supplying both silently dropped
call, supplying neither compiled and then panicked inside a render, and a
test existed only to police it.

prep does everything call did, so it is now the one spelling. The zero-arg
shapes the README already names — a derivation reads the output emitted so
far, a generator reads only the rng — lift through derive and generate, so
the registry reads as that taxonomy. iban stops looking its country up per
render.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

builtins_test.go

@@ -181,18 +181,14 @@ func ibanValid(s string) bool {
 	return rem == 1
 }
 
-// TestRegistryShapes pins the builtin contract bind relies on: exactly one of prep
-// or call, and args parsed at compile only behind a check. A registry entry with
-// neither would compile and then panic inside a render.
+// TestRegistryShapes pins the builtin contract compileOps relies on: every entry
+// supplies prep, and args parsed at compile only behind a check.
 func TestRegistryShapes(t *testing.T) {
 	for name, b := range builtins {
-		switch {
-		case b.prep == nil && b.call == nil:
-			t.Errorf("builtin %q has neither prep nor call: bind would return a nil-func closure", name)
-		case b.prep != nil && b.call != nil:
-			t.Errorf("builtin %q has both prep and call; call is dead", name)
+		if b.prep == nil {
+			t.Errorf("builtin %q has no prep: compileOps would call a nil func", name)
 		}
-		if b.prep != nil && b.arity != 0 && b.check == nil {
+		if b.arity != 0 && b.check == nil {
 			t.Errorf("builtin %q parses args in prep with no check", name)
 		}
 	}