Report a prep-time arg that its check should have caught

calcPrep discarded parseCalc's error, so a divergence from checkCalc would
have produced a nil AST and then a nil dereference per render, with no
message. atoi did the same quietly, returning zero for a length, range or
decimal count. Both now panic naming the argument, so the invariant they rest
on fails where it breaks.

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

calc.go

@@ -96,7 +96,10 @@ func checkCalc(fields map[string]node, args []string) error {
 // operand name at the position expand will read it into. checkCalc proved both args
 // valid, so no step here can fail; dp -1 prints the minimal form.
 func calcPrep(args []string) callFn {
-	expr, _ := parseCalc(args[0])
+	expr, err := parseCalc(args[0])
+	if err != nil { // a nil AST would be a nil dereference per render, with no message
+		panic(fmt.Sprintf("fakes: calc(%q) reached prep unparsed: %v", args[0], err))
+	}
 	at := make(map[string]int)
 	for i, name := range calcVars(expr) {
 		at[name] = i