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>

template.go

@@ -67,15 +67,12 @@ func eachToken(format string, fn func(ftoken) error) error {
 // seq is the one exception: it advances per-session counter state, which is itself
 // deterministic (1, 2, 3 …). arity is the exact arg count, or -1 for variadic
 // (then check does all the validation). The optional check validates args at
-// compile time (their values, beyond the count). A builtin supplies exactly one of
-// prep (args parsed once, at compile) or call (args parsed per render). The registry
-// lives in builtins.go.
+// compile time (their values, beyond the count). The registry lives in builtins.go.
 type builtin struct {
 	arity int
 	// prep parses validated args once, at compile time, into the closure expand calls.
 	prep  func(args []string) callFn
 	check func(fields map[string]node, args []string) error
-	call  func(s *session, emitted string, fields map[string]node, args []string) string
 }
 
 // funcCall splits a "{token}" body shaped name(args) into its parts; ok is false
@@ -327,7 +324,7 @@ func compileOps(format string) ([]op, int, map[string]string) {
 		case 'b':
 			flush()
 			if name, args, ok := funcCall(t.body); ok {
-				ops = append(ops, op{kind: 'b', call: builtins[name].bind(args)})
+				ops = append(ops, op{kind: 'b', call: builtins[name].prep(args)})
 			} else {
 				arms := splitArms(t.body)
 				for _, a := range arms {
@@ -348,14 +345,3 @@ func compileOps(format string) ([]op, int, map[string]string) {
 	flush()
 	return ops, grow, bound
 }
-
-// bind returns the closure for one call site, parsing args once via prep if present.
-func (b builtin) bind(args []string) callFn {
-	if b.prep != nil {
-		return b.prep(args)
-	}
-	call := b.call
-	return func(s *session, emitted string, fields map[string]node) string {
-		return call(s, emitted, fields, args)
-	}
-}