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/parser.sx

@@ -151,6 +151,8 @@
                 (do (adv!) (parse-the-expr)))
               ((and (= typ "keyword") (= val "me"))
                 (do (adv!) (list (quote me))))
+              ((and (= typ "keyword") (= val "sender"))
+                (do (adv!) (list (quote sender))))
               ((and (= typ "keyword") (= val "I"))
                 (do (adv!) (list (quote me))))
               ((and (= typ "keyword") (= val "it"))