fix(auto-thumbnail): [PKG-AUTO-THUMB-07] release terminal canvas storage

refactor/risks.json

@@ -5662,6 +5662,30 @@
       ],
       "compatibleResolution": "Isolate native media/compositor/readback and sequence effects without globally disabling default offscreen or deleting assertions. The pre-09 artifact also fails in the same sequence; do not claim the new hybrid guard caused or fixed this separate failure.",
       "closureCriteria": "Identify and verify a compatible fix or a reviewed precise platform boundary with native controls. Preserve known red sequence and successful standalone comparison; candidate playback/fullscreen pixels and lifetime must be justified, not merely rerun to green."
+    },
+    {
+      "id": "AUTO-THUMB-CANVAS-01",
+      "title": "Private thumbnail canvas retains sheet dimensions after completion or cancellation while callbacks retain the element",
+      "confirmation": "reproduced",
+      "status": "resolved",
+      "owners": [
+        "PKG-AUTO-THUMB-07"
+      ],
+      "evidence": [
+        "refactor/changes/2026-09-14-PKG-AUTO-THUMB-07-canvas.md",
+        "refactor/baselines/auto-thumbnail-canvas-validation.json",
+        "test/auto-thumbnail-canvas.test.js",
+        "test/browser/auto-thumbnail-lifecycle.spec.js"
+      ],
+      "compatibleResolution": "Own width/height reset in the extraction job and guard allocation reentry; retain completed JPEG URLs.",
+      "closureCriteria": "Controlled old/new and native retained-callback checks demonstrate terminal zero dimensions, independent cleanup and still-decodable completed JPEGs.",
+      "resolutionEvidence": [
+        "refactor/changes/2026-09-14-PKG-AUTO-THUMB-07-canvas.md",
+        "refactor/baselines/auto-thumbnail-canvas-validation.json",
+        "test/auto-thumbnail-canvas.test.js",
+        "test/browser/auto-thumbnail-lifecycle.spec.js"
+      ],
+      "resolutionRationale": "Six controlled regressions fail before and pass after. The frozen main native complete control fails dimensions; candidate main/legacy native lifecycle checks reset the retained canvas and decode final JPEGs after reset. No public options, types, sample positions or caller URLs changed; WebKit early-frame pixels remain separately open."
     }
   ]
 }