Add render tab to tree editor, switch MCP paths to SX format

Tree editor island now has 4 tabs: tree, context, validate, render.
The render tab evaluates SX source as live HTML — type a (div (h2 "Hello"))
and see it rendered immediately.

MCP server paths changed from JSON arrays [0,2,1] to SX strings "(0 2 1)".
Fixes serialization issues and is more natural for an SX tool. The
json_to_path function now parses SX via sx-parse.

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

hosts/ocaml/bin/mcp_tree.ml

@@ -174,9 +174,20 @@ let parse_file path =
   let exprs = Sx_parser.parse_all src in
   List exprs
 
+let parse_path_str s =
+  (* Parse SX path string: "(0 3 2)" or "0 3 2" → SX list of numbers *)
+  let exprs = Sx_parser.parse_all s in
+  match exprs with
+  | [List items] ->
+    (* (0 3 2) → list of numbers *)
+    List (List.map (fun x -> match x with Number _ -> x | _ -> Number 0.0) items)
+  | _ ->
+    (* Bare numbers: "0 3 2" → parsed as separate exprs *)
+    List (List.map (fun x -> match x with Number _ -> x | _ -> Number 0.0) exprs)
+
 let json_to_path j =
   let open Yojson.Safe.Util in
-  List (List.map (fun x -> Number (float_of_int (to_int x))) (to_list j))
+  parse_path_str (to_string j)
 
 let value_to_string v =
   match v with
@@ -311,7 +322,7 @@ let tool name desc props required =
       ("properties", `Assoc props)])]
 
 let file_prop = ("file", `Assoc [("type", `String "string"); ("description", `String "Path to .sx file")])
-let path_prop = ("path", `Assoc [("type", `String "array"); ("items", `Assoc [("type", `String "integer")]); ("description", `String "Index path, e.g. [0,2,1]")])
+let path_prop = ("path", `Assoc [("type", `String "string"); ("description", `String "SX path, e.g. \"(0 2 1)\"")])
 
 let tool_definitions = `List [
   tool "sx_read_tree" "Read an .sx file as an annotated tree with path labels on every node. Use this to understand structure before editing."