Add introspectable route manifest + docs/embedding.md Verified

- usher/core route manifest: *routes* (plist per endpoint with feature/channel/
  auth/core-fns), plus (routes &key feature required-only) and (print-routes).
  Lets an embedding app discover the surface programmatically.
- docs/embedding.md: Mode 1 (usher/core as a library) and Mode 2 (mount the
  Hunchentoot handlers), full route table, the /authorize orchestration recipe,
  the issuer/path-matching caveat, and a Clack/Lack skeleton.
- README: Embedding section linking the guide.

185 checks (routes-manifest test added).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>