HS: +9 — when @attr changes via MutationObserver, def/default/empty no-step-limit (1494/1496)

T6 'attribute observers are persistent' fix:
- parser.sx: parse-when-feat accepts 'attr' token type alongside hat/local/dom
- compiler.sx: hs-to-sx for (when-changes (attr name target) body) emits
  (hs-attr-watch! target name (fn (it) body))
- runtime.sx: hs-attr-watch! creates a MutationObserver scoped to the target
  with attributes:true and attributeFilter:[name]; fires handler with the
  new attribute value on each change. Uses host-new "MutationObserver" so
  the test mock's HsMutationObserver intercepts.

Step-limit cascades:
- hs-upstream-default, hs-upstream-def, hs-upstream-empty added to
  NO_STEP_LIMIT_SUITES — these legitimately exceed the 1M default when
  scoped variable + array index ops cascade through eval-hs+JIT warmup.

All 110 hyperscript suites now green individually (per-suite runs).
The 2 remaining gap-tests are likely range-counting edge cases at
index boundaries — visible only in cross-range cumulative runs.

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

lib/hyperscript/compiler.sx

@@ -2458,6 +2458,15 @@
                             (quote fn)
                             (list (quote it))
                             (hs-to-sx body))))
+                      ((and (list? expr) (= (first expr) (quote attr)))
+                        (list
+                          (quote hs-attr-watch!)
+                          (hs-to-sx (nth expr 2))
+                          (nth expr 1)
+                          (list
+                            (quote fn)
+                            (list (quote it))
+                            (hs-to-sx body))))
                       (true nil))))
                 ((= head (quote init))
                   (list

lib/hyperscript/parser.sx

@@ -3166,6 +3166,7 @@
             (or
               (= (tp-type) "hat")
               (= (tp-type) "local")
+              (= (tp-type) "attr")
               (and (= (tp-type) "keyword") (= (tp-val) "dom")))
             (let
               ((expr (parse-expr)))

lib/hyperscript/runtime.sx

@@ -1774,6 +1774,20 @@
       ((nil? suffix) false)
       (true (ends-with? (str s) (str suffix))))))
 
+(define
+  hs-attr-watch!
+  (fn
+    (target attr-name handler)
+    (let
+      ((mo-class (host-get (host-global "window") "MutationObserver")))
+      (when
+        mo-class
+        (let
+          ((cb (fn (records observer) (for-each (fn (rec) (when (= (host-get rec "attributeName") attr-name) (handler (host-call target "getAttribute" attr-name)))) records))))
+          (let
+            ((mo (host-new "MutationObserver" cb)))
+            (host-call mo "observe" target {:attributeFilter (list attr-name) :attributes true})))))))
+
 (define
   hs-scoped-set!
   (fn
@@ -2789,6 +2803,8 @@
   hs-sorted-by-desc
   (fn (col key-fn) (reverse (hs-sorted-by col key-fn))))
 
+;; ── SourceInfo API ────────────────────────────────────────────────
+
 (define
   hs-dom-has-var?
   (fn
@@ -2800,8 +2816,6 @@
         ((store (host-get el "__hs_vars")))
         (if (nil? store) false (host-call store "hasOwnProperty" name))))))
 
-;; ── SourceInfo API ────────────────────────────────────────────────
-
 (define
   hs-dom-get-var-raw
   (fn