Support pre-dispatch hook registration in app! #434

User story

As an application author using app!, I want to install a pre-dispatch hook while retaining generated routes, middleware, state, manifest introspection, stores, logging and application identity, so that reserved-namespace interception does not require duplicate router configuration.

Acceptance criteria

• Define and document an optional macro argument for hook installation, including value/Arc behavior and duplicate-registration semantics.
• Existing macro syntax and generated application behavior remain compatible.
• Tests demonstrate terminal and continuation behavior and preservation of every generated metadata/router surface.
• No runtime-specific dependencies enter core or macros.

Affected area

Macros (#[action], #[app])

Follow-up to the routing-workaround finding in #403; that PR documents and tests the explicit manual reconstruction required today.