Let hoists report emptiness instead of comparing rendered text

The while handler inferred "nothing hoisted" from render() returning
its input unchanged -- true today, but any formatting change in render()
would silently flip every plain do-while into the exit-check lowering.
new_hoist() exposes is_empty() and render() reuses it.

Review finding (fable-final-review.md #6); no behavior change.

Author: mns-nordicals

R/r2f-aab-core.R

@@ -20,6 +20,10 @@ new_hoist <- function(scope) {
 
   has_block <- function() !is.null(block_scope)
 
+  # TRUE when render(code) would return `code` unchanged: nothing emitted,
+  # no block-scoped temporaries declared.
+  is_empty <- function() !length(hoisted) && !has_block()
+
   ensure_block_scope <- function() {
     if (is.null(block_scope)) {
       block_scope <<- scope_new_child(scope, "block")
@@ -42,7 +46,7 @@ new_hoist <- function(scope) {
 
   render <- function(code) {
     code <- str_split_lines(code)
-    if (!length(hoisted) && !has_block()) {
+    if (is_empty()) {
       return(str_flatten_lines(code))
     }
 
@@ -65,6 +69,7 @@ new_hoist <- function(scope) {
     list(
       emit = emit,
       declare_tmp = declare_tmp,
+      is_empty = is_empty,
       render = render
     ),
     parent = emptyenv()

R/r2f-control-flow.R

@@ -82,9 +82,7 @@ r2f_handlers[["while"]] <- function(args, scope, ..., hoist = NULL) {
   # (`{` bodies already isolate each statement.)
   body <- r2f(args[[2]], scope, ..., hoist = NULL)
   check_pending_parallel_consumed(scope)
-  exit_check <- glue("if (.not. ({cond})) exit")
-  cond_code <- cond_hoist$render(exit_check)
-  if (identical(as.character(cond_code), as.character(exit_check))) {
+  if (cond_hoist$is_empty()) {
     # nothing hoisted: keep the plain do-while form
     return(Fortran(glue(
       "do while ({cond})
@@ -93,6 +91,7 @@ r2f_handlers[["while"]] <- function(args, scope, ..., hoist = NULL) {
       "
     )))
   }
+  cond_code <- cond_hoist$render(glue("if (.not. ({cond})) exit"))
   Fortran(glue(
     "do
     {indent(cond_code)}