fix(web-ui): compose Meme panel through Workbench

[kachofugetsu09]

Summary

• register the Meme dashboard through workbench.panels.v2
• preserve the established category and gallery layout
• cancel stale category and image reads when selection or ownership changes
• keep the module installable through the ordinary immutable plugin artifact path
• return root.unmount() through the shared Workbench mount/disposer lifecycle

Depends on kachofugetsu09/akashic-agent#520.

Validation

• final HEAD: 404494d21ecfa21eb003bac7078d9ce05dcfb5fb
• contract and full 14-test composition parity CI passed against the pinned Web UI Host SDK Core
• controlled browser E2E proves a delayed old image read cannot replace the active category
• Core artifact resolver accepted the real module and stylesheet