Step 14: source locations — pos-to-loc, error-loc, sx-parse-loc — 15 tests

Pure SX layer: pos-to-loc (offset→line/col), error-loc (parse result→loc),
format-parse-error (human-readable error with source context line).
OCaml platform: cst_to_ast_loc (CST spans→loc dicts), sx-parse-loc
primitive (parse with locations), source-loc accessor.

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

hosts/ocaml/bin/run_tests.ml

@@ -164,6 +164,24 @@ let make_test_env () =
     | [String s] -> List (parse_all s)
     | _ -> raise (Eval_error "sx-parse: expected string"));
 
+  bind "sx-parse-loc" (fun args ->
+    match args with
+    | [String s] ->
+      let cst = Sx_parser.parse_all_cst s in
+      List (Sx_cst.cst_to_ast_loc s cst.nodes)
+    | _ -> raise (Eval_error "sx-parse-loc: expected string"));
+
+  bind "source-loc" (fun args ->
+    match args with
+    | [Dict d] ->
+      let line = try Hashtbl.find d "line" with Not_found -> Nil in
+      let col = try Hashtbl.find d "col" with Not_found -> Nil in
+      let ld = Sx_types.make_dict () in
+      Sx_types.dict_set ld "line" line;
+      Sx_types.dict_set ld "col" col;
+      Dict ld
+    | _ -> Nil);
+
   bind "sx-parse-one" (fun args ->
     match args with
     | [String s] ->