feat(ext-openapi): pick the page that reads the document

Swagger UI, Redoc or Scalar, each configured with its own options under the
names its own documentation gives them, or no page at all. What a UI does not
name still passes through, so a bundle's whole option set stays reachable
without this extension tracking it.

The page is rendered once at boot and the document is encoded once per
revision, so a request to any of the three routes hands out bytes rather than
building them: the spec used to be serialized again on every single request.

/openapi/swagger becomes /openapi/docs, because the path names what is served
and not which bundle happens to serve it. That page also named a preset that
lives in a bundle it never loaded, and BaseLayout never needed it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

flash-extensions/flash-ext-limiter/docs/http-headers.md

@@ -115,7 +115,7 @@ and **instead of** calling it on rejected requests. This means:
   avoid this by not setting these headers manually).
 - On 429, the handler body is never executed — no side effects occur.
 
-## Integration with Swagger UI (flash-ext-openapi)
+## Integration with OpenAPI (flash-ext-openapi)
 
 When `flash-ext-openapi` is installed, handlers annotated with `@Limit` automatically
 contribute rate-limit response headers to generated OpenAPI responses: