experiment: desktop v2 tab architecture

[Hona]

Summary

• split legacy and Desktop V2 application roots around canonical V2 routes
• make tabs navigation records over one stable session host instead of mounted applications
• scope server, directory, draft, notification, permission, terminal, and async operation ownership explicitly
• preserve timeline/composer ownership across cold same-server and cross-server tab switches

Validation

• repository pre-push typecheck: 22 packages passed
• bun typecheck in packages/app
• bun typecheck in packages/ui
• bun test src/context/terminal.test.ts: 6 passed
• git diff --check

Status

Draft experiment. Full Desktop runtime verification across the multi-server tab matrix is still required. The branch is currently based 6 commits behind dev.