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_primitives.ml

@@ -30,7 +30,7 @@ let as_number = function
   | Bool false -> 0.0
   | Nil -> 0.0
   | String s -> (match float_of_string_opt s with Some n -> n | None -> Float.nan)
-  | v -> raise (Eval_error ("Expected number, got " ^ type_of v))
+  | v -> raise (Eval_error ("Expected number, got " ^ type_of v ^ ": " ^ (match v with Dict d -> (match Hashtbl.find_opt d "__signal" with Some _ -> "signal{value=" ^ (match Hashtbl.find_opt d "value" with Some v' -> value_to_string v' | None -> "?") ^ "}" | None -> "dict") | _ -> "")))
 
 let as_string = function
   | String s -> s