Fix VM correctness: get nil-safe, scope/context/collect! as primitives

- get primitive returns nil for type mismatches (list+string) instead
  of raising — matches JS/Python behavior, fixes find-nav-match errors
- scope-peek, collect!, collected, clear-collected! registered as real
  primitives in sx_primitives table (not just env bindings) so the CEK
  step-sf-context can find them via get-primitive
- step-sf-context checks scope-peek hashtable BEFORE walking CEK
  continuation — bridges aser's scope-push!/pop! with CEK's context
- context, emit!, emitted added to SPECIAL_FORM_NAMES and handled in
  aser-special (scope operations in aser rendering mode)
- sx-context NativeFn for VM-compiled code paths
- VM execution errors no longer mark functions as permanently failed —
  bytecode is correct, errors are from runtime data
- kbd, samp, var added to HTML_TAGS + sx-browser.js rebuilt

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

hosts/ocaml/lib/sx_primitives.ml

@@ -500,7 +500,9 @@ let () =
     | [Dict d; Keyword k] -> dict_get d k
     | [List l; Number n] | [ListRef { contents = l }; Number n] ->
       (try List.nth l (int_of_float n) with _ -> Nil)
-    | _ -> raise (Eval_error "get: dict+key or list+index"));
+    | [Nil; _] -> Nil  (* nil.anything → nil *)
+    | [_; _] -> Nil    (* type mismatch → nil (matches JS/Python behavior) *)
+    | _ -> Nil);
   register "has-key?" (fun args ->
     match args with
     | [Dict d; String k] -> Bool (dict_has d k)