HS: append command, list stringify as comma-separated

Compiler:
- append to symbol → (set! target (hs-append target value))
- append to DOM → (hs-append! value target)

Runtime:
- hs-append: pure function for string concat and list append
- hs-append!: DOM insertAdjacentHTML for element targets

Mock DOM:
- dom_stringify handles List by joining elements with commas
  (matching JS Array.toString() behavior)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/hyperscript/compiler.sx

@@ -1143,10 +1143,13 @@
               ((= head (quote go))
                 (list (quote hs-navigate!) (hs-to-sx (nth ast 1))))
               ((= head (quote append!))
-                (list
-                  (quote dom-append)
-                  (hs-to-sx (nth ast 2))
-                  (hs-to-sx (nth ast 1))))
+                (let
+                  ((tgt (hs-to-sx (nth ast 2)))
+                    (val (hs-to-sx (nth ast 1))))
+                  (if
+                    (symbol? tgt)
+                    (list (quote set!) tgt (list (quote hs-append) tgt val))
+                    (list (quote hs-append!) val tgt))))
               ((= head (quote tell))
                 (let
                   ((tgt (hs-to-sx (nth ast 1))))