Gate client well-known lookups on enableClientWellKnownLookups
New URL param (default on), threaded from the embedder. When off, always
avoid client .well-known: skip the post-login poll (widget + SPA) and the
legacy MatrixRTC foci fallback. When on/unset, respect the transports
endpoint response: only fall back to .well-known on a 404 M_UNRECOGNIZED.
