HS: send can reference sender (+1 test)

Three-part fix: (a) emit-send now builds detail=(dict "sender" me) on
(send NAME target) and bare (send NAME) instead of nil, so the receiving
handler has access to the sending element. (b) parser parse-atom now
recognises the `sender` keyword (previously swallowed as noise) and
emits it as (sender). (c) compiler translates bare `sender` symbol and
(sender) list-head to (hs-sender event) — a new runtime helper that
reads (get (host-get event "detail") "sender").

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

lib/hyperscript/compiler.sx

@@ -264,8 +264,17 @@
                 name
                 (hs-to-sx (nth ast 2))))
             ((= (len ast) 3)
-              (list (quote dom-dispatch) (hs-to-sx (nth ast 2)) name nil))
-            (true (list (quote dom-dispatch) (quote me) name nil))))))
+              (list
+                (quote dom-dispatch)
+                (hs-to-sx (nth ast 2))
+                name
+                (list (quote dict) "sender" (quote me))))
+            (true
+              (list
+                (quote dom-dispatch)
+                (quote me)
+                name
+                (list (quote dict) "sender" (quote me))))))))
     (define
       emit-repeat
       (fn
@@ -650,11 +659,15 @@
         ((number? ast) ast)
         ((string? ast) ast)
         ((boolean? ast) ast)
+        ((and (symbol? ast) (= (str ast) "sender"))
+          (list (quote hs-sender) (quote event)))
         ((not (list? ast)) ast)
         (true
           (let
             ((head (first ast)))
             (cond
+              ((= head (quote sender))
+                (list (quote hs-sender) (quote event)))
               ((= head (quote null-literal)) nil)
               ((= head (quote not))
                 (list (quote not) (hs-to-sx (nth ast 1))))