Decouple core evaluator from web platform, extract libraries

The core evaluator (spec/evaluator.sx) is now the irreducible computational
core with zero web, rendering, or type-system knowledge. 2531 → 2313 lines.

- Add extensible special form registry (*custom-special-forms* + register-special-form!)
- Add render dispatch hooks (*render-check* / *render-fn*) replacing hardcoded render-active?/is-render-expr?/render-expr
- Extract freeze scopes → spec/freeze.sx (library, not core)
- Extract content addressing → spec/content.sx (library, not core)
- Move sf-deftype/sf-defeffect → spec/types.sx (self-registering)
- Move sf-defstyle → web/forms.sx (self-registering with all web forms)
- Move web tests (defpage, streaming) → web/tests/test-forms.sx
- Add is-else-clause? helper (replaces 5 inline patterns)
- Make escape-html/escape-attr library functions in render.sx (pure SX, not platform-provided)
- Add foundations plan: Step 3.5 (data representations), Step 3.7 (verified components), OCaml for Step 4d
- Update all three bootstrappers (JS 957/957, Python 744/744, OCaml 952/952)

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

hosts/ocaml/lib/sx_runtime.ml

@@ -297,10 +297,26 @@ let scope_pop _name = Nil
 let provide_push name value = ignore name; ignore value; Nil
 let provide_pop _name = Nil
 
-(* Render mode stubs *)
-let render_active_p () = Bool false
-let render_expr _expr _env = Nil
-let is_render_expr _expr = Bool false
+(* Custom special forms registry — mutable dict *)
+let custom_special_forms = Dict (Hashtbl.create 4)
+
+(* register-special-form! — add a handler to the custom registry *)
+let register_special_form name handler =
+  (match custom_special_forms with
+   | Dict tbl -> Hashtbl.replace tbl (value_to_str name) handler; handler
+   | _ -> raise (Eval_error "custom_special_forms not a dict"))
+
+(* Render check/fn hooks — nil by default, set by platform if needed *)
+let render_check = Nil
+let render_fn = Nil
+
+(* is-else-clause? — check if a cond/case test is an else marker *)
+let is_else_clause v =
+  match v with
+  | Keyword k -> Bool (k = "else" || k = "default")
+  | Symbol s -> Bool (s = "else" || s = "default")
+  | Bool true -> Bool true
+  | _ -> Bool false
 
 (* Signal accessors *)
 let signal_value s = match s with Signal sig' -> sig'.s_value | _ -> raise (Eval_error "not a signal")