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>

MAINTENANCE.md

@@ -698,6 +698,26 @@ as a commit co-author, and this repository follows the same rule.
 Each dependency carries its own `AGENTS.md` with authoritative per-repo rules. Read the
 relevant one before editing a submodule.
 
+### Abandoned generators
+
+The one invariant generators (`GEN`/`EMIT`, `FOR EACH`/`DO EACH`) add to the environment
+pool: **a suspended generator hangs off its loop scope's `forGeneratorEnv` as a *child*,
+off the parent chain, so no walk up `->parent` ever finds it.** Any path that discards a
+loop scope — `EXIT`, a mismatched `NEXT`, a `LOOP` condition saying stop, an error unwind
+through `akbasic_runtime_pump_generator()` or `akbasic_runtime_call_function()` — must
+release that generator too, or its pool slot is stranded until the next `RUN`; with
+twelve slots, a loop that abandons a dozen of them kills the program with "Environment
+pool exhausted" far from the cause.
+
+Two functions own the invariant, and every discard path goes through one of them:
+`akbasic_runtime_release_generator()` (walks a detached generator chain up through its
+call frame, recursing into any `forGeneratorEnv` it passes) and
+`akbasic_runtime_unwind_to_environment()` (pops the *active* chain down to a target,
+releasing each popped scope's suspended generator on the way). If you add a new path
+that pops or discards environments, use one of these — a bare
+`akbasic_runtime_prev_environment()` loop reintroduces the leak, and
+`tests/generators.c` holds the pool-exhaustion tests that will say so.
+
 ---
 
 ## Editing the documentation