HS: parser+compiler — toggle for-in lookahead, throttled/debounced modifiers (-2 skips)

parser.sx parse-toggle-cmd: when seeing 'toggle .foo for', peek the
following two tokens. If they are '<ident> in', it is a for-in loop
and toggle does NOT consume 'for' as a duration clause. Restores the
trailing for-in to the command list.

parser.sx parse-on (handler modifiers): recognize 'throttled at <ms>'
and 'debounced at <ms>' as handler modifiers. Captured as :throttle /
:debounce kwargs in the on-form parts list.

compiler.sx emit-on: pre-extract :throttle / :debounce from parts via
new _strip-throttle-debounce helper before scan-on, then wrap the built
handler with (hs-throttle! handler ms) or (hs-debounce! handler ms).

runtime.sx: hs-throttle! — closure with __hs-last-fire timestamp,
fires immediately and drops events arriving within ms of the last fire.
hs-debounce! — closure with __hs-timer, clears any pending timer and
schedules a new setTimeout(handler, ms) so only the last burst event
fires.

Both formerly-architectural skips now pass:
- "toggle does not consume a following for-in loop"
- "throttled at <time> drops events within the window"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/hyperscript/runtime.sx

@@ -54,6 +54,41 @@
   hs-on-every
   (fn (target event-name handler) (dom-listen target event-name handler)))
 
+;; Throttle: drops events that arrive within the window. First event fires
+;; immediately; subsequent events within `ms` of the previous fire are dropped.
+;; Returns a wrapped handler suitable for hs-on / hs-on-every.
+(define
+  hs-throttle!
+  (fn
+    (handler ms)
+    (let
+      ((__hs-last-fire 0))
+      (fn
+        (event)
+        (let
+          ((__hs-now (host-call (host-global "Date") "now")))
+          (when
+            (>= (- __hs-now __hs-last-fire) ms)
+            (set! __hs-last-fire __hs-now)
+            (handler event)))))))
+
+;; Debounce: waits until `ms` has elapsed since the last event before firing.
+;; In our synchronous test mock no time passes, so the timer fires immediately
+;; via setTimeout(_, 0); the wrapped handler still gets called once per burst.
+(define
+  hs-debounce!
+  (fn
+    (handler ms)
+    (let
+      ((__hs-timer nil))
+      (fn
+        (event)
+        (when __hs-timer (host-call (host-global "window") "clearTimeout" __hs-timer))
+        (set! __hs-timer
+          (host-call (host-global "window") "setTimeout"
+            (host-new-function (list "ev") "return arguments[0](arguments[1]);")
+            ms handler event))))))
+
 ;; Wait for a DOM event on a target.
 ;; (hs-wait-for target event-name) — suspends until event fires
 (define