HS: assignableElements — set vs put distinction (+8 tests)

Parser: parse-set-cmd now emits (set-el! target value) when target is
a query node (e.g. #id, .class), keeping (set! ...) for all other
targets.

Compiler: add (set-el! ...) handler that calls hs-set-element!; revert
emit-set for query targets back to hs-set-inner-html! so that
put "x" into #target keeps setting innerHTML rather than replacing
the element.

Runtime: hs-set-element! new function — parses value as HTML into a
temp div; if it contains element children, replaces the target element
via replaceChild and boots hyperscript on the new element; otherwise
falls through to hs-set-inner-html!. Removes the spurious
host-to-list wrapper that was causing len() to always return 0.

Result: all 8 assignableElements tests pass (set #id / set .class /
set closest / swap, plus put-into-still-works-as-innerHTML).

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

lib/hyperscript/runtime.sx

@@ -314,6 +314,23 @@
       (let
         ((str-val (if (list? value) (join "" (map (fn (x) (str x)) value)) value)))
         (do (dom-set-inner-html target str-val) (hs-boot-subtree! target)))))
+  (define
+    hs-set-element!
+    (fn
+      (target value)
+      (let ((parent (dom-parent target)))
+        (when parent
+          (let ((tmp (dom-create-element "div"))
+                (str-val (if (list? value) (join "" (map (fn (x) (str x)) value)) value)))
+            (do
+              (dom-set-inner-html tmp str-val)
+              (let ((children (host-get tmp "children")))
+                (if (> (len children) 0)
+                  (let ((new-el (first children)))
+                    (do
+                      (host-call parent "replaceChild" new-el target)
+                      (hs-boot-subtree! new-el)))
+                  (hs-set-inner-html! target str-val)))))))))
   (define
     hs-put!
     (fn