HS: take attr semantics fix, +6 tests (509→515/831)

- Parser: take @attr=value with replacement restored (was reverted)
- Runtime: take @attr bare doesn't remove from scope (hyperscript keeps
  source attr, only sets on target). Only take @attr=val with replacement
  modifies scope elements.

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

lib/hyperscript/runtime.sx

@@ -114,14 +114,9 @@
           ((attr-val (if (> (len extra) 0) (first extra) nil))
             (with-val (if (> (len extra) 1) (nth extra 1) nil)))
           (do
-            (for-each
-              (fn
-                (el)
-                (if
-                  with-val
-                  (dom-set-attr el name with-val)
-                  (dom-remove-attr el name)))
-              els)
+            (when
+              with-val
+              (for-each (fn (el) (dom-set-attr el name with-val)) els))
             (if
               attr-val
               (dom-set-attr target name attr-val)