Middleware: FetchHandler

The type of a composed stack handed to the host — the fetch export, Deno.serve(app), and so on.

Annotating the outermost handler with this (… satisfies FetchHandler) is optional and adds no runtime code. It does two things:

• Asserts the stack can be the fetch export. Prerequisites are enforced between layers with no annotation. What is left over is the case where no layer supplies a declared In key: the requirement is republished all the way out, so the stack has a required ctx. That alone is not an error — a required ctx is only wrong where the stack is handed to the host, and this type is what makes that position explicit (its ctx is optional, and a required one is not assignable to it). An untyped export default { fetch: app } checks nothing, so without the annotation the stack compiles, ships, and reads undefined off ctx on the first request. Any FetchHandler-typed position does the same job.
• Turns on collision detection. Two middleware contributing the same key are only caught under this annotation; see the Middleware overload set. One annotation on the outermost call covers any nesting depth.

It is not needed for accumulation: an unannotated outermost call resolves Base to its constraint — the empty upstream, the same context an annotation would seed — so the cascade reaches the innermost handler at any depth either way. Nor for In prerequisites (those travel outward), nor for pipeline (which accumulates and validates from its entries array).