Island SSR: defislands render to HTML server-side with hydration markers

Islands now render their initial state as HTML on the server, like
React SSR. The client hydrates with reactive behavior on boot.

Root causes fixed:
- is_signal/signal_value now recognize Dict-based signals (from
  signals.sx) in addition to native Signal values
- Register "context" as a primitive so the CEK deref frame handler
  can read scope stacks for reactive tracking
- Load adapter-html.sx into kernel for SX-level render-to-html
  (islands use this instead of the OCaml render module)
- Component accessors (params, body, has-children?, affinity) handle
  Island values with ? suffix aliases
- Add platform primitives: make-raw-html, raw-html-content,
  empty-dict?, for-each-indexed, cek-call

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

hosts/ocaml/lib/sx_runtime.ml

@@ -346,7 +346,10 @@ let is_else_clause v =
   | _ -> Bool false
 
 (* Signal accessors *)
-let signal_value s = match s with Signal sig' -> sig'.s_value | _ -> raise (Eval_error "not a signal")
+let signal_value s = match s with
+  | Signal sig' -> sig'.s_value
+  | Dict d -> (match Hashtbl.find_opt d "value" with Some v -> v | None -> Nil)
+  | _ -> 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