WIP: pre-existing changes from WASM browser work + test infrastructure

Accumulated changes from WASM browser development sessions:
- sx_runtime.ml: signal subscription + notify, env unwrap tolerance
- sx_browser.bc.js: rebuilt js_of_ocaml browser kernel
- sx_browser.bc.wasm.js + assets: WASM browser kernel build
- sx-platform.js browser tests (test_js, test_platform, test_wasm)
- Playwright sx-inspect.js: interactive page inspector tool
- harness-web.sx: DOM assertion updates
- deploy.sh, Dockerfile, dune-project: build config updates
- test-stepper.sx: stepper unit tests
- reader-macro-demo plan update

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

hosts/ocaml/lib/sx_runtime.ml

@@ -361,11 +361,44 @@ let signal_value s = match s with
   | _ -> raise (Eval_error "not a signal")
 let signal_set_value s v = match s with Signal sig' -> sig'.s_value <- v; v | _ -> raise (Eval_error "not a signal")
 let signal_subscribers s = match s with Signal sig' -> List (List.map (fun _ -> Nil) sig'.s_subscribers) | _ -> List []
-let signal_add_sub_b _s _f = Nil
-let signal_remove_sub_b _s _f = Nil
-let signal_deps _s = List []
-let signal_set_deps _s _d = Nil
-let notify_subscribers _s = Nil
+let signal_add_sub_b s f =
+  match s with
+  | Dict d ->
+    (match Hashtbl.find_opt d "subscribers" with
+     | Some (ListRef r) -> r := !r @ [f]; Nil
+     | Some (List items) -> Hashtbl.replace d "subscribers" (ListRef (ref (items @ [f]))); Nil
+     | _ -> Hashtbl.replace d "subscribers" (ListRef (ref [f])); Nil)
+  | _ -> Nil
+
+let signal_remove_sub_b s f =
+  match s with
+  | Dict d ->
+    (match Hashtbl.find_opt d "subscribers" with
+     | Some (ListRef r) -> r := List.filter (fun x -> x != f) !r; Nil
+     | Some (List items) -> Hashtbl.replace d "subscribers" (List (List.filter (fun x -> x != f) items)); Nil
+     | _ -> Nil)
+  | _ -> Nil
+
+let signal_deps s =
+  match s with
+  | Dict d -> (match Hashtbl.find_opt d "deps" with Some v -> v | None -> List [])
+  | _ -> List []
+
+let signal_set_deps s d =
+  match s with
+  | Dict tbl -> Hashtbl.replace tbl "deps" d; Nil
+  | _ -> Nil
+
+let notify_subscribers s =
+  let subs = match s with
+    | Dict d -> (match Hashtbl.find_opt d "subscribers" with
+       | Some (ListRef { contents = items }) | Some (List items) -> items
+       | _ -> [])
+    | _ -> []
+  in
+  List.iter (fun f -> ignore (sx_call f [])) subs;
+  Nil
+
 let flush_subscribers _s = Nil
 let dispose_computed _s = Nil