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] ->

hosts/ocaml/lib/sx_cst.ml

@@ -104,6 +104,33 @@ let rec cst_to_ast = function
     Dict d
 
 
+(** Convert character offset to line/col (1-based lines, 0-based cols) *)
+let offset_to_loc src offset =
+  let line = ref 1 and col = ref 0 in
+  for i = 0 to min (offset - 1) (String.length src - 1) do
+    if src.[i] = '\n' then (incr line; col := 0)
+    else col := !col + 1
+  done;
+  (!line, !col)
+
+(** CST → AST with source location dicts ({:form value :line N :col N}) *)
+let cst_to_ast_loc src nodes =
+  List.map (fun node ->
+    let span = match node with
+      | CstAtom { span; _ } -> span
+      | CstList { span; _ } -> span
+      | CstDict { span; _ } -> span
+    in
+    let value = cst_to_ast node in
+    let (line, col) = offset_to_loc src span.start_offset in
+    let d = make_dict () in
+    dict_set d "form" value;
+    dict_set d "line" (Number (float_of_int line));
+    dict_set d "col" (Number (float_of_int col));
+    Dict d
+  ) nodes
+
+
 (** {1 CST editing — apply AST-level edits back to the CST} *)
 
 (** Replace the CST node at [path] with [new_source], preserving the