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/parser_commands.c

@@ -337,6 +337,19 @@ akerr_ErrorContext *akbasic_parse_do(akbasic_Parser *parser, akbasic_ASTLeaf **d
 
 	    PASS(errctx, parse_each_clause(parser, &var, &callexpr));
 
+	    /*
+	     * Same guard as akbasic_parse_for()'s EACH branch, with the likely
+	     * mistake named: a condition belongs on the LOOP, where it is
+	     * checked against each emitted value, not here on the DO.
+	     */
+	    peeked = akbasic_parser_peek(parser);
+	    FAIL_NONZERO_RETURN(errctx,
+				(peeked != NULL &&
+				 peeked->tokentype != AKBASIC_TOK_UNDEFINED &&
+				 peeked->tokentype != AKBASIC_TOK_COLON),
+				AKBASIC_ERR_SYNTAX,
+				"DO EACH takes its WHILE/UNTIL on the LOOP, and nothing else here");
+
 	    newenv->isDoLoop = true;
 	    newenv->isEachLoop = true;
 	    newenv->loopFirstLine = firstline;
@@ -1098,6 +1111,21 @@ akerr_ErrorContext *akbasic_parse_for(akbasic_Parser *parser, akbasic_ASTLeaf **
 
 	    PASS(errctx, parse_each_clause(parser, &var, &callexpr));
 
+	    /*
+	     * Nothing may follow the generator call but another statement.
+	     * Without this, a stray clause sits unparsed on the line and only
+	     * blows up after the whole loop has run, when the parent scope
+	     * resumes the line mid-statement -- an error at the loop's end
+	     * pointing at its beginning.
+	     */
+	    peeked = akbasic_parser_peek(parser);
+	    FAIL_NONZERO_RETURN(errctx,
+				(peeked != NULL &&
+				 peeked->tokentype != AKBASIC_TOK_UNDEFINED &&
+				 peeked->tokentype != AKBASIC_TOK_COLON),
+				AKBASIC_ERR_SYNTAX,
+				"FOR EACH takes nothing after the generator call");
+
 	    newenv->isEachLoop = true;
 	    newenv->loopFirstLine = firstline;
 	    /*