Task 2 of the Slint POC readiness audit. AppRuntime now performs
framework-agnostic per-user session activation: activate, activate_at,
activate_offline, deactivate, plus is_session_active, active_user_id,
and a with_session_store accessor.
This is the minimal version per the 2026-05-18 audit correction. It
opens only the session store and does the portable scaffolding (user
paths, directories, last_user_id marker, runtime state machine). It
does not touch Tauri's session_lifecycle, does not initialize the
src-tauri-side per-user stores, and does not run the flat-to-user
migration. The shipping app keeps identical behavior; this is additive.
activate_at takes explicit directories and is the testable core;
activate resolves real per-user paths through UserDataPaths.
cc
vicrodh
7ef6300a22