Adds a framework-agnostic composition root for non-Tauri UI shells
(Slint, TUI, headless) as Task 1 of the Slint POC readiness audit
(session-20-slint-poc-readiness-audit).
AppRuntime<A> owns an Arc<QbzCore<A>> plus the runtime state machine,
built with the same recipe as the Tauri CoreBridge but with no Tauri
dependency. Scope is composition and accessors only; per-user store
ownership and session activation are deferred to Task 2.
qbz-app now depends on qbz-core, qbz-player, qbz-audio, and qbz-models.
The Tauri app does not consume this module; it is purely additive.
cc
vicrodh
a213b37807