Refactor _control() by extracting per-loop handlers and stack/iter helper

Add _eval_expr_list() to isolate condition evaluation in a temporary
stack+iterator, restoring the outer stack before returning.  This
eliminates the duplicate stack/iter backup boilerplate that existed
separately in the while/until and do-while paths.

Extract four static handler functions from the 280-line _control() body:
  _control_if_ifnot()    — TT_IF / TT_IFNOT
  _control_while_until() — TT_WHILE / TT_UNTIL; uses _eval_expr_list
  _control_loop()        — TT_LOOP
  _control_do()          — TT_DO; uses _eval_expr_list for post-condition

_control() itself shrinks to a ~30-line dispatcher (ret/break/next inline;
loops and conditionals delegated to their handlers).

Semantic improvement in while/until: the original code ran the body with
a temp stack active and then did a manual stack_merge for CONTROL_RET.
Since _eval_expr_list now restores the outer stack before the body runs,
interpret_subprocess merges return values into the outer stack directly —
the extra stack_merge is no longer needed.

Also removes the leftover debug printf("FOO")/exit(0) guard.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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