HS: when :count changes — scoped watch + parse-cmd feature boundary fix

Three-part fix for element-scoped reactive expressions:

1. Parser: add when/bind to parse-cmd's feature-keyword nil set so
   `... then when X changes ...` is parsed as a new feature, not absorbed
   into the preceding on-handler body as a (ref "when") expression.

2. Parser: parse-when-feat now recognises local (:var) token type so
   `when :count changes ...` dispatches to the when-changes branch.

3. Runtime + compiler: hs-scoped-set! now fires hs-scoped-fire-watchers!
   on change; new hs-scoped-watch! / hs-scoped-fire-watchers! registry;
   compiler emits (hs-scoped-watch! me name (fn (it) body)) for local
   expressions in when-changes AST nodes.

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

lib/hyperscript/runtime.sx

@@ -1686,7 +1686,34 @@
 
 (define
   hs-scoped-set!
-  (fn (el name val) (dom-set-data el (str "hs-local-" name) val)))
+  (fn
+    (el name val)
+    (let
+      ((changed (not (= (hs-scoped-get el name) val))))
+      (do
+        (dom-set-data el (str "hs-local-" name) val)
+        (when changed (hs-scoped-fire-watchers! el name val))))))
+
+(begin
+  (define _hs-scoped-watchers (list))
+  (define
+    hs-scoped-watch!
+    (fn
+      (el name handler)
+      (set!
+        _hs-scoped-watchers
+        (cons (list el name handler) _hs-scoped-watchers))))
+  (define
+    hs-scoped-fire-watchers!
+    (fn
+      (el name val)
+      (for-each
+        (fn
+          (entry)
+          (when
+            (and (= (nth entry 0) el) (= (nth entry 1) name))
+            ((nth entry 2) val)))
+        _hs-scoped-watchers))))
 
 (define
   hs-scoped-get
@@ -2701,6 +2728,8 @@
               (if match (dom-parent match) nil)))
           (true el))))))
 
+;; ── SourceInfo API ────────────────────────────────────────────────
+
 (define
   hs-dom-walk
   (fn
@@ -2711,8 +2740,6 @@
       ((= (dom-get-attr el "dom-scope") "isolated") nil)
       (true (hs-dom-walk (dom-parent el) name)))))
 
-;; ── SourceInfo API ────────────────────────────────────────────────
-
 (define
   hs-dom-find-owner
   (fn