Add asset cleanup preview and orphaned file cleanup

[errorsallthethings]

Summary

• replace the prune-only maintenance action with an Asset Cleanup dry-run preview
• show unreferenced assets, orphaned campaign asset files, stale thumbnail references, retained asset counts, and removable file totals before deletion
• remove orphaned files from campaign asset folders during cleanup while keeping deletion scoped to campaign-owned asset paths
• update cleanup result reporting, docs, and release smoke checklist

Notes

• Cleanup applies to campaign asset files and thumbnail references. Effects are scene metadata today, not separate asset files.

Verification

• npm.cmd test -- assetCleanupPlanning assetMaintenanceServices assetMaintenanceIpc gmMaintenanceState campaignMaintenance
• npm.cmd run check
• npm.cmd run smoke