[SDK-6e] Extension point: ITargetLifecycleListener

[ChrisonSimtian]

Expose ITargetLifecycleListener as a public extension point for lifecycle callbacks. Maps to surface 5 in RFC #2.

Scope

• Callbacks: target running / succeeded / failed / build finished.
• Replaces the existing IOnTargetRunning, IOnTargetSucceeded, etc. IBuildExtension hooks (kept internal in v11, promoted here).
• Multiple listeners run in registration order — no priority ordering needed.

Depends on

SDK-1, v11 #94 (IBuildExtension refactor onto middleware).