mediator: MediatorBuilder and in-process dispatch

[pierrick-fonquerne]

Implement the mediator that routes commands and queries to their registered handlers through the middleware pipeline.

Scope

• MediatorBuilder fluent builder: .with_handler(h), .with_middleware(m), .with_middleware_for::<C, M>(m), .verify_handlers(), .build().
• Mediator with send::<C> and query::<Q> dispatching through the global then per-type middleware chain.
• Handler registry backed by inventory for compile-time type discovery.
• build() returns Err when verify_handlers() detects a registered type with no instance.

Acceptance criteria

• Mediator::builder().with_handler(h).build() dispatches a command to the correct handler.
• A global middleware executes for every send and query call.
• A per-type middleware executes only for the targeted command type.
• build() returns Err(HandlerNotFound) when verify_handlers() is active and an instance is missing.
• Integration test exercises a full chain: middleware to handler to result.