Spec explorer: fix SxExpr rendering bugs, add drill-in UX, Playwright tests

Fix 3 OCaml bugs that caused spec explorer to hang:
- sx_types: inspect outputs quoted string for SxExpr (not bare symbol)
- sx_primitives: serialize/to_string extract SxExpr/RawHTML content
- sx_render: handle SxExpr in both render-to-html paths

Restructure spec explorer for performance:
- Lightweight overview: name + kind only (was full source for 141 defs)
- Drill-in detail: click definition → params, effects, signature
- explore() page function accepts optional second arg for drill-in
- spec() passes through non-string slugs from nested routing

Fix aser map result wrapping:
- aser-special map now wraps results in fragment (<> ...) via aser-fragment
- Prevents ((div ...) (div ...)) nested lists that caused client "Not callable"

5 Playwright tests: overview load, no errors, SPA nav, drill-in detail+params

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

hosts/ocaml/lib/sx_render.ml

@@ -201,6 +201,7 @@ let rec do_render_to_html (expr : value) (env : env) : string =
   | String s -> escape_html s
   | Keyword k -> escape_html k
   | RawHTML s -> s
+  | SxExpr s -> s
   | Symbol s ->
     let v = Sx_ref.eval_expr (Symbol s) (Env env) in
     do_render_to_html v env
@@ -280,7 +281,12 @@ and render_list_to_html head args env =
        | _ ->
          let result = Sx_ref.eval_expr (List (head :: args)) (Env env) in
          do_render_to_html result env)
-    with Eval_error _ -> "")
+    with Eval_error _ ->
+      (* Primitive or special form — not in env, delegate to CEK *)
+      (try
+        let result = Sx_ref.eval_expr (List (head :: args)) (Env env) in
+        do_render_to_html result env
+      with Eval_error _ -> ""))
   | _ ->
     let result = Sx_ref.eval_expr (List (head :: args)) (Env env) in
     do_render_to_html result env
@@ -456,6 +462,7 @@ let rec render_to_buf buf (expr : value) (env : env) : unit =
   | String s -> escape_html_buf buf s
   | Keyword k -> escape_html_buf buf k
   | RawHTML s -> Buffer.add_string buf s
+  | SxExpr s -> Buffer.add_string buf s
   | Symbol s ->
     let v = Sx_ref.eval_expr (Symbol s) (Env env) in
     render_to_buf buf v env