feat(background): runner PTY B8 + boucle sink fermée + refactor point-2

Livre le lot backend B8 des tâches de fond :

- infrastructure : runner de commandes concret (CommandBackgroundRunner)
  sur le port BackgroundTaskRunner, tail borné (bounded_tail/BoundedTail),
  éclatement du module background_task en sous-modules (mod/runner/tail,
  sink extrait de l'ancien background_task.rs).
- application : nouveau module background exposant les cas d'usage
  SpawnBackgroundCommand, CancelBackgroundTask, RetryBackgroundTask et le
  port BackgroundCommandArchive.
- domain : refactor point-2 de l'arbitrage Architect — sortie du trait
  BackgroundCommandArchive de la couche domaine vers application.
- app-tauri : câblage runtime (commands, dto, state, lib) des commandes
  spawn/cancel/retry et de la boucle de complétion sink fermée en
  composition root.

Build workspace + tests application/infrastructure verts (QA).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/infrastructure/src/background_task/mod.rs

@@ -0,0 +1,22 @@
+//! First-class background task infrastructure.
+//!
+//! Three cohesive pieces close the B2→B8 loop:
+//!
+//! - [`sink`] — the durable [`BackgroundCompletionSink`], single writer between a
+//!   runner completion and the ready-to-deliver signal (persist-before-signal).
+//! - [`tail`] — a UTF-8-safe bounded ring buffer for stdout/stderr tails.
+//! - [`runner`] — [`CommandBackgroundRunner`], the concrete
+//!   [`BackgroundTaskRunner`](domain::ports::BackgroundTaskRunner) that spawns a
+//!   command-backed task over a composed [`PtyPort`](domain::ports::PtyPort) and
+//!   emits exactly one completion per task.
+
+mod runner;
+mod sink;
+mod tail;
+
+pub use runner::CommandBackgroundRunner;
+pub use sink::{
+    start_background_ready_inbox_bridge, BackgroundCompletionSink, BackgroundCompletionSinkError,
+    BackgroundCompletionSinkOutcome, BackgroundReadyInboxBridgeHandle, BackgroundTaskReadyToDeliver,
+};
+pub use tail::{bounded_tail, tail_cap_bytes, BoundedTail};