Fix DOM adapter: filter spread values from dom-append calls

Spread values returned by components like ~cssx/tw are not DOM nodes
and cannot be passed to appendChild. Filter them in fragment, let,
begin/do, component children, and data list rendering paths — matching
the HTML adapter's existing spread filtering.

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

shared/sx/ref/adapter-dom.sx

@@ -160,7 +160,11 @@
         ;; Data list
         :else
           (let ((frag (create-fragment)))
-            (for-each (fn (x) (dom-append frag (render-to-dom x env ns))) expr)
+            (for-each (fn (x)
+              (let ((result (render-to-dom x env ns)))
+                (when (not (spread? result))
+                  (dom-append frag result))))
+              expr)
             frag)))))
 
 
@@ -296,10 +300,14 @@
           (component-params comp))
 
         ;; If component accepts children, pre-render them to a fragment
+        ;; Spread values are filtered out (no parent element to merge onto)
         (when (component-has-children? comp)
           (let ((child-frag (create-fragment)))
             (for-each
-              (fn (c) (dom-append child-frag (render-to-dom c env ns)))
+              (fn (c)
+                (let ((result (render-to-dom c env ns)))
+                  (when (not (spread? result))
+                    (dom-append child-frag result))))
               children)
             (env-set! local "children" child-frag)))
 
@@ -314,7 +322,10 @@
   (fn ((args :as list) (env :as dict) (ns :as string))
     (let ((frag (create-fragment)))
       (for-each
-        (fn (x) (dom-append frag (render-to-dom x env ns)))
+        (fn (x)
+          (let ((result (render-to-dom x env ns)))
+            (when (not (spread? result))
+              (dom-append frag result))))
         args)
       frag)))
 
@@ -498,24 +509,32 @@
       (= name "case")
         (render-to-dom (trampoline (eval-expr expr env)) env ns)
 
-      ;; let / let*
+      ;; let / let* — single body: pass through (spread propagates). Multi: fragment.
       (or (= name "let") (= name "let*"))
-        (let ((local (process-bindings (nth expr 1) env))
-              (frag (create-fragment)))
-          (for-each
-            (fn (i)
-              (dom-append frag (render-to-dom (nth expr i) local ns)))
-            (range 2 (len expr)))
-          frag)
+        (let ((local (process-bindings (nth expr 1) env)))
+          (if (= (len expr) 3)
+            (render-to-dom (nth expr 2) local ns)
+            (let ((frag (create-fragment)))
+              (for-each
+                (fn (i)
+                  (let ((result (render-to-dom (nth expr i) local ns)))
+                    (when (not (spread? result))
+                      (dom-append frag result))))
+                (range 2 (len expr)))
+              frag)))
 
-      ;; begin / do
+      ;; begin / do — single body: pass through. Multi: fragment.
       (or (= name "begin") (= name "do"))
-        (let ((frag (create-fragment)))
-          (for-each
-            (fn (i)
-              (dom-append frag (render-to-dom (nth expr i) env ns)))
-            (range 1 (len expr)))
-          frag)
+        (if (= (len expr) 2)
+          (render-to-dom (nth expr 1) env ns)
+          (let ((frag (create-fragment)))
+            (for-each
+              (fn (i)
+                (let ((result (render-to-dom (nth expr i) env ns)))
+                  (when (not (spread? result))
+                    (dom-append frag result))))
+              (range 1 (len expr)))
+            frag))
 
       ;; Definition forms — eval for side effects
       (definition-form? name)