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>

src/runtime_commands.c

@@ -161,8 +161,22 @@ akerr_ErrorContext *akbasic_cmd_return(akbasic_Runtime *obj, akbasic_ASTLeaf *ex
 	SUCCEED_TRUE(obj, dest);
 	SUCCEED_RETURN(errctx);
     }
+    /*
+     * A GEN is a function at heart, and RETURN ends it the way it ends a DEF
+     * or a GOSUB: early, cleanly, from its own frame. What a generator's
+     * RETURN cannot do is carry a value -- values leave a GEN one at a time,
+     * through EMIT, and there is no caller waiting on a return slot.
+     */
+    if ( obj->environment->isGenerator ) {
+	FAIL_NONZERO_RETURN(errctx, (expr != NULL && expr->right != NULL), AKBASIC_ERR_STATE,
+			    "A GEN yields values through EMIT; RETURN here takes none");
+	PASS(errctx, akbasic_runtime_prev_environment(obj));
+	obj->environment->forGeneratorEnv = NULL;
+	SUCCEED_TRUE(obj, dest);
+	SUCCEED_RETURN(errctx);
+    }
     FAIL_ZERO_RETURN(errctx, (obj->environment->gosubReturnLine != 0), AKBASIC_ERR_STATE,
-		     "RETURN outside the context of GOSUB");
+		     "RETURN outside the context of GOSUB, DEF, or GEN");
 
     if ( expr != NULL && expr->right != NULL ) {
 	PASS(errctx, akbasic_runtime_evaluate(obj, expr->right, &result));