refactor(qbz-app): add AppRuntime shell facade skeleton

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

crates/qbz-app/Cargo.toml

@@ -6,6 +6,10 @@ license.workspace = true
 repository.workspace = true
 
 [dependencies]
+qbz-models = { path = "../qbz-models" }
+qbz-audio = { path = "../qbz-audio" }
+qbz-player = { path = "../qbz-player" }
+qbz-core = { path = "../qbz-core" }
 serde = { workspace = true }
 serde_json = { workspace = true }
 tokio = { workspace = true }