Walks the router's registered route table -- method + path template +
path-parameter names -- and builds a spec whose paths group by
template with one operation per method. Path parameters (:name
segments) surface as required path parameters; every operation gets a
default 200 response.
ponytail: this derives the structural surface the runtime Router
retains (methods, path templates, path params). Request/response body
schemas and query/header parameters live on the typed Extracted[H]
handler, which the Router erases at registration -- deriving those needs
a comptime walk of reflect[H] at the registration site (the
ComptimeRouter path), not runtime introspection. Mounted sub-routers are
not walked yet (top-level routes only).