Fix morph-children: treat empty-string element IDs as nil

dom-id returns "" for elements without an id attribute, and "" is
truthy in SX. This caused morph-children to build spurious entries
in old-by-id keyed by "", then match unrelated children via the
empty key — advancing oi past id-keyed children like #main-content
and skipping them in cleanup.

Three changes in morph-children (engine.sx):
- old-by-id reduce: normalize empty dom-id to nil so id-less
  elements are excluded from the lookup dict
- match-id binding: normalize empty dom-id to nil so new children
  without ids don't spuriously match old children
- Case 2 skip condition: use (not (empty? ...)) instead of bare
  (dom-id old-child) to avoid treating "" as a real id

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

shared/static/wasm/sx/engine.sx

@@ -416,7 +416,7 @@
             (fn
               ((acc :as dict) kid)
               (let
-                ((id (dom-id kid)))
+                ((id (let ((raw (dom-id kid))) (if (empty? raw) nil raw))))
                 (if id (do (dict-set! acc id kid) acc) acc)))
             (dict)
             old-kids))
@@ -425,7 +425,7 @@
         (fn
           (new-child)
           (let
-            ((match-id (dom-id new-child))
+            ((match-id (let ((raw-id (dom-id new-child))) (if (empty? raw-id) nil raw-id)))
               (match-by-id (if match-id (dict-get old-by-id match-id) nil)))
             (cond
               (and match-by-id (not (nil? match-by-id)))
@@ -444,7 +444,7 @@
               (let
                 ((old-child (nth old-kids oi)))
                 (if
-                  (and (dom-id old-child) (not match-id))
+                  (and (not (empty? (dom-id old-child))) (not match-id))
                   (dom-insert-before
                     old-parent
                     (dom-clone new-child)