[SDK-6a] Extension point: IBuildMiddleware

[ChrisonSimtian]

Expose IBuildMiddleware as a public extension point. Maps to surface 1 in RFC #2.

Scope

• Make IBuildMiddleware and its phases public (Initializing / DiscoveringTargets / Planning / Executing / Reporting).
• Registration via IServiceCollection.AddFalloutMiddleware<T>(phase, priority) or similar.
• The v11 internal pipeline ([Foundation] Replace init-pipeline attributes with internal IBuildMiddleware #93, [Foundation] Refactor IBuildExtension hooks onto middleware pipeline #94) is the host-side implementation; v12 promotes the contract.
• Validate priority/phase contract in conflict resolution (SDK-4).

Depends on

v11: #93, #94. v12: SDK-1.