IdentitySwitchService

Mid-session identity switch orchestrator (§SP-8).

Detailed description

Phase 10a scaffolding — emits the §IS-1 signal surface and walks the §IS-2 five (six) phases in order, but the phase bodies are TODO stubs. Phase 11 wires them to real subsystems per §SP-8a.

The current implementation is intentionally synchronous on the UI thread (§IS-9): the placeholder phase bodies have no async work to await. Phase 11 introduces QFutureWatcher-based phase awaits as the individual subsystem flush() and mount() calls come online.

Spec: content/doc/internal/specs/startup-identity-unification/identity-switch.md