desktop: implement the desktop seam set against the conformance kits

[FSM1]

Scope

The desktop seam implementations under <data_local_dir>/cipherbox//, ciphertext-only at rest: FloorStore (fsync-barriered files), StagingStore (generalized v1 write journal: one durable record per op, sidecar staged ciphertext, json-before-bin removal, orphan-sidecar GC), SnapshotCache (sealed cache files), CredentialStore (keyring; refresh token + last-account id only), Http/RecordTransport (reqwest), Scheduler (Tokio).
Does NOT construct the engine at login or touch the webview.

Blueprint anchors

blueprint/desktop.md: Engine wiring

Gate

Seam conformance kits for the desktop implementations inside the cargo workspace tests PR gate.

Depends on

• desktop: scaffold the apps/desktop Tauri shell and version surface #645 (desktop: scaffold the apps/desktop Tauri shell and version surface)
• engine: scaffold seams, fakes, conformance kits, and the facade #623 (engine: scaffold seams, fakes, conformance kits, and the facade)