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/bin/sx_server.ml

@@ -125,6 +125,51 @@ let io_batch_mode = ref false
 let io_queue : (int * string * value list) list ref = ref []
 let io_counter = ref 0
 
+(** Module-level scope stacks — shared between make_server_env (aser
+    scope-push!/pop!) and step-sf-context (via get-primitive "scope-peek"). *)
+let _scope_stacks : (string, value list) Hashtbl.t = Hashtbl.create 8
+
+let () = Sx_primitives.register "scope-peek" (fun args ->
+  match args with
+  | [String name] ->
+    let stack = try Hashtbl.find _scope_stacks name with Not_found -> [] in
+    (match stack with v :: _ -> v | [] -> Nil)
+  | _ -> Nil)
+
+(** collect! — lazy scope accumulator. Creates root scope if missing,
+    emits value (deduplicates).  Used by cssx and spread components. *)
+let () = Sx_primitives.register "collect!" (fun args ->
+  match args with
+  | [String name; value] ->
+    let stack = try Hashtbl.find _scope_stacks name with Not_found -> [] in
+    (match stack with
+     | List items :: rest ->
+       if not (List.mem value items) then
+         Hashtbl.replace _scope_stacks name (List (items @ [value]) :: rest)
+     | [] ->
+       (* Lazy root scope — create with the value *)
+       Hashtbl.replace _scope_stacks name [List [value]]
+     | _ :: _ -> ());
+    Nil
+  | _ -> Nil)
+
+let () = Sx_primitives.register "collected" (fun args ->
+  match args with
+  | [String name] ->
+    let stack = try Hashtbl.find _scope_stacks name with Not_found -> [] in
+    (match stack with List items :: _ -> List items | _ -> List [])
+  | _ -> List [])
+
+let () = Sx_primitives.register "clear-collected!" (fun args ->
+  match args with
+  | [String name] ->
+    let stack = try Hashtbl.find _scope_stacks name with Not_found -> [] in
+    (match stack with
+     | _ :: rest -> Hashtbl.replace _scope_stacks name (List [] :: rest)
+     | [] -> ());
+    Nil
+  | _ -> Nil)
+
 (** Helpers safe to defer — pure functions whose results are only used
     as rendering output (inlined into SX wire format), not in control flow. *)
 let batchable_helpers = [
@@ -309,8 +354,9 @@ let make_server_env () =
   bind "render-active?" (fun _args -> Bool true);
 
   (* Scope stack — platform primitives for render-time dynamic scope.
-     Used by aser for spread/provide/emit patterns. *)
-  let scope_stacks : (string, value list) Hashtbl.t = Hashtbl.create 8 in
+     Used by aser for spread/provide/emit patterns.
+     Module-level so step-sf-context can check it via get-primitive. *)
+  let scope_stacks = _scope_stacks in
   bind "scope-push!" (fun args ->
     match args with
     | [String name; value] ->
@@ -346,6 +392,19 @@ let make_server_env () =
        | [] -> ()); Nil
     | _ -> Nil);
 
+  (* context — scope lookup.  The CEK handles this as a special form
+     by walking continuation frames, but compiled VM code needs it as
+     a function that reads from the scope_stacks hashtable. *)
+  bind "sx-context" (fun args ->
+    match args with
+    | [String name] | [String name; _] ->
+      let stack = try Hashtbl.find scope_stacks name with Not_found -> [] in
+      (match stack, args with
+       | v :: _, _ -> v
+       | [], [_; default_val] -> default_val
+       | [], _ -> Nil)
+    | _ -> Nil);
+
   (* Evaluator bridge — aser calls these spec functions.
      Route to the OCaml CEK machine. *)
   bind "eval-expr" (fun args ->
@@ -714,9 +773,10 @@ let register_jit_hook env =
     | Lambda l ->
       (match l.l_compiled with
        | Some cl when not (Sx_vm.is_jit_failed cl) ->
-         (* Cached bytecode — execute on VM, fall back to CEK on error *)
+         (* Cached bytecode — execute on VM, fall back to CEK on error.
+            Don't invalidate cache — bytecode is correct, error is runtime. *)
          (try Some (Sx_vm.call_closure cl args cl.vm_env_ref)
-          with _ -> l.l_compiled <- Some Sx_vm.jit_failed_sentinel; None)
+          with _ -> None)
        | Some _ -> None  (* failed sentinel *)
        | None ->
          (* Don't try to compile while already compiling (prevents