HS toggle style: parse between/cycle, runtime, mock style dict

Parser:
- Reorder toggle style parsing: target before between clause
- Handle "indexed" keyword, "indexed by" syntax
- Use parse-atom (not parse-expr) for between values to avoid
  consuming "and" as boolean operator
- Support 3-4 value cycles via toggle-style-cycle

Compiler:
- Add toggle-style-cycle dispatch → hs-toggle-style-cycle!

Runtime:
- Add hs-toggle-style-between! (2-value toggle)
- Add hs-toggle-style-cycle! (N-value round-robin)

Mock DOM:
- Parse CSS strings from setAttribute "style" into style sub-dict
  so dom-get-style/dom-set-style work correctly

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

hosts/ocaml/bin/run_tests.ml

@@ -1815,6 +1815,22 @@ let run_spec_tests env test_files =
                 Hashtbl.replace d "className" (String sv);
               end;
               if name = "disabled" then Hashtbl.replace d "disabled" (Bool true);
+              if name = "style" then begin
+                (* Parse CSS string into the style sub-dict *)
+                let style_d = match Hashtbl.find_opt d "style" with Some (Dict s) -> s | _ ->
+                  let s = Hashtbl.create 4 in Hashtbl.replace d "style" (Dict s); s in
+                let parts = String.split_on_char ';' sv in
+                List.iter (fun part ->
+                  let part = String.trim part in
+                  if String.length part > 0 then
+                    match String.index_opt part ':' with
+                    | Some i ->
+                      let prop = String.trim (String.sub part 0 i) in
+                      let value = String.trim (String.sub part (i+1) (String.length part - i - 1)) in
+                      Hashtbl.replace style_d prop (String value)
+                    | None -> ()
+                ) parts
+              end;
               Nil
             | _ -> Nil)
          | "getAttribute" ->