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>

TODO.md

@@ -366,6 +366,36 @@ One caveat survives the upgrade unchanged: `aksl_strhash_djb2` still sign-extend
 high-bit byte hashes differently from the `unsigned char` answer. BASIC identifiers are 7-bit
 ASCII so the symbol tables cannot reach it — see §1.3, which is still accurate.
 
+### 1.10 Generators share the `DEF` namespace, and the rest of their v1 semantics
+
+Decided with issue #57 and its review. `GEN` and `DEF` live in the same functions table —
+one lookup, one "unknown function" error path, a name cannot be both. What follows from
+that and from the review of PR #61, all settled:
+
+- A `GEN` called like a function (`X# = COUNTUP(3)`) is refused at the call, before
+  anything is pushed — `akbasic_FunctionDef.isGenerator` exists for exactly this check.
+- Bare `RETURN` standing in a `GEN`'s own frame ends the generator exactly as `END GEN`
+  does; `RETURN expr` there is an error, because values leave a `GEN` only through `EMIT`.
+  Both inherit the interpreter-wide restriction that `RETURN` does not unwind nested
+  `FOR`/`DO` scopes — lifting that everywhere (the C64 reference *does* unwind) is filed
+  separately.
+- `DO EACH ... LOOP WHILE c | UNTIL c` composes: the condition is checked after each trip,
+  with the loop variable still holding that trip's value, and stopping abandons the
+  generator exactly as `EXIT` does. A condition on the `DO EACH` line itself is a parse
+  error.
+- Self-recursion — a `GEN` reached again down its own parent chain — is refused; sibling
+  and nested invocations of the same `GEN` are each a fresh pool environment and are fine.
+- Every path that discards a `FOR EACH`/`DO EACH` scope must release the generator
+  suspended off it; see MAINTENANCE.md's "Abandoned generators" note for the invariant
+  and `akbasic_runtime_unwind_to_environment()` for the one primitive that enforces it.
+
+One known limitation, pre-existing and shared with `DEF`: `parse_def_parameters()` does
+not accept an empty parameter list, so `GEN NAME()` cannot be written — every generator
+takes at least one parameter whether it wants one or not. Location:
+`src/parser_commands.c`, `parse_def_parameters()`. Consequence: pointless parameters in
+programs. Blast radius: cosmetic, both `DEF` and `GEN` headers. Closure: teach the shared
+helper to accept `()`, one test each for `DEF` and `GEN`; filed as its own issue.
+
 ---
 
 ## 2. What exists — **the core port is complete and green**