HS: toggle multi-class + until event (+2 tests)

Parser `parse-toggle-cmd`: after the leading class ref, collect any
additional class refs and treat `toggle .foo .bar` as `toggle-between`
(pair-only). Recognise a `until EVENT [from SOURCE]` modifier and emit
a new `toggle-class-until` AST node. Compiler handles the new node by
emitting `(begin (hs-toggle-class! tgt cls) (hs-wait-for src ev)
(hs-toggle-class! tgt cls))` which uses the existing event-waiter
machinery to flip the class back when the specified event fires.

Remaining toggle test (`can toggle for a fixed amount of time`)
depends on the mock's sync io-sleep resuming immediately — the click
handler toggles on/off synchronously, so the pre-timeout assertion
can never see the `.foo` class present. Needs an async scheduler in
the mock to handle.

Suite hs-upstream-toggle: 22/25 → 24/25. Smoke 0-195: 162/195
unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/hyperscript/compiler.sx

@@ -1426,6 +1426,20 @@
                     (quote hs-toggle-class!)
                     (hs-to-sx (nth ast 2))
                     (nth ast 1))))
+              ((= head (quote toggle-class-until))
+                (let
+                  ((cls (nth ast 1))
+                    (tgt (hs-to-sx (nth ast 2)))
+                    (event-name (nth ast 3))
+                    (source (nth ast 4)))
+                  (list
+                    (quote do)
+                    (list (quote hs-toggle-class!) tgt cls)
+                    (list
+                      (quote hs-wait-for)
+                      (if source (hs-to-sx source) (quote me))
+                      event-name)
+                    (list (quote hs-toggle-class!) tgt cls))))
               ((= head (quote set-on))
                 (list
                   (quote hs-set-on!)