Lightpanda upstream merged the AX-tree nodeId fix in
lightpanda-io/browser#2232. The TestLightpanda_KnownUpstreamGaps canary
fired on the latest image, so:
- Rename to TestLightpanda_Snapshot_AndEval and assert the snapshot
returns refs + non-empty text (instead of asserting it fails).
- Flip AX-snapshot in the compatibility matrix from gap to fully
supported. Replace the "Known upstream gap" section with a
"Minimum Lightpanda version" note pointing at the upstream PR.