Summary

• repair desktop project-tree indexes for legacy Global sessions whose session metadata already has a topic ID but whose sidebar topic index is missing
• preserve explicit topic deletion by recording deleted topic tombstones so repaired Global topics do not reappear after the user removes them
• add regression coverage for the repaired Global topic path and the delete-does-not-resurrect behavior

Root cause

Some upgraded desktop states can contain Global session metadata with topic_id and topic_title already set, while desktop-projects.json and the Global topic title index are missing the matching topic entries. The legacy migration skipped those sessions because they already had topic metadata, leaving the sidebar with no Global section when another project existed.

Validation

• go test . -run 'Test(ProjectTreeRepairsIndexedGlobalTopicsAfterMigrationMarker|DeletedRepairedGlobalTopicIsNotAutoRestored|RestoreGlobalTopicSessionReindexesProjectTree|RestoreProjectTopicSessionReindexesProjectTree|TrashTopicMovesRelatedSessionsToTrash|DeleteTopicKeepsSessionHistory)'

Note: a full desktop package run currently hits an existing TempDir cleanup failure in TestTurnDonePersistsSession, unrelated to this topic-index change.