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_primitives.ml

@@ -65,6 +65,8 @@ let rec to_string = function
   | Symbol s -> s
   | Keyword k -> k
   | Thunk _ as t -> to_string (!trampoline_hook t)
+  | SxExpr s -> s
+  | RawHTML s -> s
   | v -> inspect v
 
 let () =
@@ -126,10 +128,23 @@ let () =
       Number (Float.max lo (Float.min hi x))
     | _ -> raise (Eval_error "clamp: 3 args"));
   register "parse-int" (fun args ->
+    let parse_leading_int s =
+      let len = String.length s in
+      let start = ref 0 in
+      let neg = len > 0 && s.[0] = '-' in
+      if neg then start := 1
+      else if len > 0 && s.[0] = '+' then start := 1;
+      let j = ref !start in
+      while !j < len && s.[!j] >= '0' && s.[!j] <= '9' do incr j done;
+      if !j > !start then
+        let n = int_of_string (String.sub s !start (!j - !start)) in
+        Some (if neg then -n else n)
+      else None
+    in
     match args with
-    | [String s] -> (match int_of_string_opt s with Some n -> Number (float_of_int n) | None -> Nil)
+    | [String s] -> (match parse_leading_int s with Some n -> Number (float_of_int n) | None -> Nil)
     | [String s; default_val] ->
-      (match int_of_string_opt s with Some n -> Number (float_of_int n) | None -> default_val)
+      (match parse_leading_int s with Some n -> Number (float_of_int n) | None -> default_val)
     | [Number n] | [Number n; _] -> Number (float_of_int (int_of_float n))
     | [_; default_val] -> default_val
     | _ -> Nil);
@@ -276,7 +291,17 @@ let () =
         else if String.sub haystack i nl = needle then Number (float_of_int i)
         else find (i + 1)
       in find 0
-    | _ -> raise (Eval_error "index-of: 2 string args"));
+    | [List items; target] | [ListRef { contents = items }; target] ->
+      let eq a b = match a, b with
+        | String x, String y -> x = y | Number x, Number y -> x = y
+        | Symbol x, Symbol y -> x = y | Keyword x, Keyword y -> x = y
+        | Bool x, Bool y -> x = y | Nil, Nil -> true | _ -> a == b in
+      let rec find i = function
+        | [] -> Nil
+        | h :: _ when eq h target -> Number (float_of_int i)
+        | _ :: tl -> find (i + 1) tl
+      in find 0 items
+    | _ -> raise (Eval_error "index-of: 2 string args or list+target"));
   register "substring" (fun args ->
     match args with
     | [String s; Number start; Number end_] ->
@@ -655,6 +680,8 @@ let () =
     match args with [a] -> String (inspect a) | _ -> raise (Eval_error "inspect: 1 arg"));
   register "serialize" (fun args ->
     match args with
+    | [SxExpr s] -> String s
+    | [RawHTML s] -> String s
     | [a] -> String (inspect a)  (* used for dedup keys in compiler *)
     | _ -> raise (Eval_error "serialize: 1 arg"));
   register "make-symbol" (fun args ->