Evaluate a public AppShell extension interface

[BumpyClock]

Goal

Decide whether AppShell needs a public extension interface after at least three out-of-repo applications adopt the declaration model.

Entry criteria

• Three external adopters have shipped or completed migrations.
• Each unmet need is documented and cannot be expressed through AppDeclaration, declaration helpers, SetupModule, LaunchSpec, or runtime domain interfaces.

Scope

Compare real adopter needs, define the smallest stable seam if justified, and keep lifecycle phases, internal plugins, and manager storage private unless evidence requires otherwise.