core: Middleware pipeline (Middleware, Next, MessageEnvelope)

[pierrick-fonquerne]

Provide the extension point that lets users and built-in components intercept every dispatch without coupling to the handler type.

Scope

• MessageEnvelope carrying type_name, MessageId and CorrelationId for middleware inspection.
• BoxOutput type alias (Box<dyn Any + Send + Sync>) for type-erased handler output inside the pipeline.
• Next<'a> opaque struct wrapping the remainder of the middleware chain.
• Middleware async trait via trait_variant::make(Send) receiving (&MessageEnvelope, &HandlerContext, Next).

Acceptance criteria

• A custom middleware that logs before and after dispatch compiles and executes correctly.
• Next::run propagates HexeractError from the inner handler unchanged.
• A middleware short-circuiting without calling next.run returns its own error without panicking.
• cargo clippy -p hexeract-core -- -D warnings passes.