fix(Augmenter): report a response component named after a status code

[DerManoMann]

Overview

A response that fails to nest into its operation disappears from the document with nothing
said, in the default configuration.

Response::$response carries two meanings: the HTTP status code when the response is nested
in an operation, and the component name when it sits in the responses bucket. isRoot()
cannot separate them — it is true whenever the key is set. So a response that does not nest
becomes a component named after its status code, nothing references it, Augmenter\Cleanup
removes it as unused, and the author's response is gone without a diagnostic.

A reusable response named 200 is a failed merge every time, and it is the only part of
this a user can see. Reporting it turns a silent drop into a pointer.

Changes

• Augmenter\Cleanup warns when the unreferenced component it removes is a Response whose
  key looks like a status code, naming the source location. What gets removed is unchanged.
• Keying the check to removal keeps it precise: an unreferenced reusable response is ordinary
  — a library may declare more than any one document uses — so only a status-code-shaped key
  is reported, and a deliberately named component that is referenced stays silent.
• The status-code pattern moves onto Spec\Response, so the compiler's RESPONSE_KEY and
  the augmenter share one copy instead of carrying the regular expression twice.
• Four tests in CleanupTest covering the status code, a range, default, a named component
  removed silently, and a referenced status-code key that is kept.

Note

With cleanup.enabled set to false nothing warns, because the component survives into the
output where it is at least visible. Extending the compiler's validateResponses() to the
components bucket would cover that case too, but the compiler cannot see whether a component
is referenced, so it would also report a deliberately named 200 that an operation $refs.
That is part of the wider question of whether a component key may double as a value, which
also governs PathItem and MediaType, and is better settled once than guessed at here.