HS add/append: Set dedup, @attr support, when-clause result tracking

- runtime hs-add-to!/hs-append: dedupe on list targets (Set semantics)
- compiler emit-set: set result to X now syncs it too
- compiler append!: handle (local)/(ref) targets via emit-set so scoped
  vars get rebound to the returned list
- parser add/remove: accept bare @attr (not just [@attr])
- parser add-attr: support when-clause → emits add-attr-when
- compiler add-class-when/add-attr-when: collect matched items into
  the-result / it so subsequent "if the result is empty" works

+6 upstream tests in early range (add 13→17, append 10→12).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/js/test262-scoreboard.json

@@ -1,35 +1,76 @@
 {
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-    "fail": 1,
-    "skip": 0,
-    "timeout": 7,
-    "total": 8,
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+    "fail": 230,
+    "skip": 39,
+    "timeout": 2,
+    "total": 327,
+    "runnable": 288,
+    "pass_rate": 19.4
   },
   "categories": [
     {
       "category": "built-ins/Math",
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-      "pass": 0,
-      "fail": 1,
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+      "fail": 230,
+      "skip": 39,
+      "timeout": 2,
+      "pass_rate": 19.4,
       "top_failures": [
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+          62
+        ],
+        [
+          "TypeError: not a function",
+          46
+        ],
         [
           "SyntaxError (parse/unsupported syntax)",
-          1
+          35
+        ],
+        [
+          "Unhandled: Unexpected token: op '++'\\",
+          3
         ]
       ]
     }
   ],
   "top_failure_modes": [
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+      46
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     [
       "SyntaxError (parse/unsupported syntax)",
+      35
+    ],
+    [
+      "Unhandled: Unexpected token: op '++'\\",
+      3
+    ],
+    [
+      "Timeout",
+      2
+    ],
+    [
+      "Unhandled: Not callable: {:random <js-math-random()> :floor <js-math-fl",
       1
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 40.1
+  "elapsed_seconds": 185.3
 }