Unify scope mechanism: one world (hashtable stacks everywhere)

Replace continuation-based scope frames with hashtable stacks for all
scope operations. The CEK evaluator's scope/provide/context/emit!/emitted
now use scope-push!/pop!/peek/emit! primitives (registered in
sx_primitives table) instead of walking continuation frames.

This eliminates the two-world problem where the aser used hashtable
stacks (scope-push!/pop!) but eval-expr used continuation frames
(ScopeFrame/ScopeAccFrame). Now both paths share the same mechanism.

Benefits:
- scope/context works inside eval-expr calls (e.g. (str ... (context x)))
- O(1) scope lookup vs O(n) continuation walking
- Simpler — no ScopeFrame/ScopeAccFrame/ProvideFrame creation/dispatch
- VM-compiled code and CEK code both see the same scope state

Also registers scope-push!/pop!/peek/emit!/collect!/collected/
clear-collected! as real primitives (sx_primitives table) so the
transpiled evaluator can call them directly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/transpiler.sx

@@ -120,6 +120,8 @@
   "emitted" "sx_emitted"
   "scope-push!" "scope_push"
   "scope-pop!" "scope_pop"
+  "scope-peek" "scope_peek"
+  "scope-emit!" "scope_emit"
   "provide-push!" "provide_push"
   "provide-pop!" "provide_pop"
   "sx-serialize" "sx_serialize"