Fix generator teardown leaks, add RETURN-in-GEN and LOOP conditions on DO EACH

Review findings and follow-ups from PR #61 review:

- runtime_generator.c: akbasic_runtime_release_generator() now releases the
  forGeneratorEnv of every scope it walks through. Abandoning a generator
  that was itself suspended inside a FOR EACH over another generator
  stranded the inner generator's pool slot; a loop doing so exhausted the
  twelve-slot pool and died far from the cause.
- runtime.c/runtime.h: new akbasic_runtime_unwind_to_environment(), the
  shared teardown for the error unwinds in pump_generator() and
  call_function() -- both previously bare prev_environment() loops with the
  same suspended-generator blindness.
- runtime_commands.c: bare RETURN standing in a GEN's own frame ends the
  generator exactly as END GEN does -- a GEN is a function at heart. RETURN
  with a value there is refused (values leave a GEN only through EMIT). The
  no-frame error message now says "GOSUB, DEF, or GEN".
- runtime_structure.c: LOOP WHILE/UNTIL composes with DO EACH -- checked
  after each trip with the loop variable still holding that trip's value; a
  condition that stops the loop abandons the generator exactly as EXIT
  does. Previously the condition was silently ignored, while the verb
  reference documented it as working.
- parser_commands.c: trailing tokens after the generator call on a FOR
  EACH/DO EACH line are refused at parse. Previously they sat unparsed and
  blew up only after the loop completed, when the parent scope resumed the
  line mid-statement -- an error at the loop's end pointing at its start.
- tests/generators.c: pool-exhaustion tests for the nested-abandonment and
  LOOP-condition paths, RETURN semantics tests, and a direct test of the
  unwind primitive. Three new golden pairs cover RETURN, LOOP conditions
  and the misplaced-condition parse error.
- docs: RETURN and LOOP-condition semantics in 04-control-flow.md and
  11-verb-reference.md; corrected the self-recursion analogy (functions
  are re-entrant here). TODO.md 1.10 records the generator design
  decisions the code comments were already citing, plus the zero-arg
  parameter-list limitation. MAINTENANCE.md gains the abandoned-generators
  invariant those comments also cited.

Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

include/akbasic/runtime.h

@@ -635,6 +635,28 @@ akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_detach_environment(akbasic_Ru
  */
 akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_release_environment(akbasic_Runtime *obj, akbasic_Environment *env);
 
+/**
+ * @brief Pop and release every scope from the active one up to @p target.
+ *
+ * The shared teardown for every path that abandons part of the environment
+ * chain at once instead of popping it a verb at a time: the error unwinds in
+ * akbasic_runtime_pump_generator() and akbasic_runtime_call_function(). Each
+ * popped scope's suspended generator (`forGeneratorEnv`), if it still holds
+ * one, is released through akbasic_runtime_release_generator() -- a suspended
+ * generator is a *child* of its loop scope, so no walk up the parent chain
+ * would ever reach it.
+ *
+ * Stops without error at the root if @p target is not on the chain: callers
+ * are already cleaning up after a failure, and releasing everything is the
+ * least-wrong answer to a target that has gone missing.
+ *
+ * @param obj Object to initialize, inspect, or modify.
+ * @param target The scope to stop at; it is left active and untouched.
+ * @return `NULL` on success, otherwise an error context owned by the caller.
+ * @throws AKERR_NULLPOINTER When @p target is NULL.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_unwind_to_environment(akbasic_Runtime *obj, akbasic_Environment *target);
+
 /**
  * @brief Push a GEN's environment, bind its parameters and run it to its first EMIT.
  *