forth: TCO at colon-def endings (no extra frame on tail-call ops)

plans/forth-on-sx.md

@@ -99,13 +99,23 @@ Representation:
 
 ### Phase 6 — speed
 - [x] Inline primitive calls during compile (skip dict lookup)
-- [ ] Tail-call optimise colon-def endings
+- [x] Tail-call optimise colon-def endings
 - [ ] JIT cooperation: mark compiled colon-defs as VM-eligible
 
 ## Progress log
 
 _Newest first._
 
+- **Phase 6 — TCO at colon-def endings (Hayes unchanged at 618/638).**
+  `forth-run-body` now special-cases the final op when it's a plain
+  function (not a branch dict): we call it in tail position with no
+  pc-increment and no recursive `forth-run-body` call. This means
+  the SX CEK can collapse the continuation frame, so chains like
+  `: A ... B ; : B ... C ; …` and `RECURSE` deep-recursion test
+  cases run without piling up frames at each colon-def boundary.
+  All 306 internal tests still green; verified 5000-deep
+  `COUNTDOWN RECURSE` still terminates fine.
+
 - **Phase 6 — inline primitive calls (Hayes unchanged at 618/638).**
   `forth-compile-call` now appends the looked-up word's body fn
   directly to the colon-def body instead of wrapping it in