spec: add SPEC-013 Windows Session Selection and Backstage Mode

docs/FEATURE_ROADMAP.md

@@ -62,6 +62,7 @@ Bringing GC to parity with GuruRMM's release engineering. Full plan: [SPEC-001](
 - [x] Protobuf-over-WSS transport, Zstd frame compression
 - [~] React/TS web viewer (`dashboard/src/components/RemoteViewer.tsx`) — embeddable session viewer
 - [ ] **Headless Linux mode (direct TTY access)** — P2 — Terminal-based remote access for Linux servers without GUI. PTY spawn (`openpty`), xterm.js web viewer, full ANSI/VT100 support. Enables server management, container debugging, emergency recovery via GuruConnect dashboard with audit logging. SSH replacement with centralized auth. ([SPEC-012](specs/SPEC-012-headless-linux-tty.md))
+- [ ] **Windows session selection and backstage mode** — P2 — Enumerate and switch between Windows user sessions (Terminal Services/RDP/Fast User Switching) and access Session 0 (backstage) for system-level admin tasks. ScreenConnect parity: session selector shows all logged-on users, instant switching without reconnect. Backstage mode provides terminal/command interface for services management without disrupting any user desktop. Critical for multi-user server environments. ([SPEC-013](specs/SPEC-013-session-selection-and-backstage.md))
 - [ ] Multi-monitor switching — P2
 - [ ] File transfer — P3 (out of scope for native-remote-control v1)
 - [ ] Session recording — P3 (out of scope for native-remote-control v1)