row 74 F25: the door path's userinfo Basic is a root-held auth credential; the kernel holds only the binding

[MattJackson]

Stacked on #922, which is stacked on #913 and #876. This is a DRAFT per queue rule 19 until its bases land.

Why

ARCHITECT ruling (docs/design/BUSBAR-1.6.0.md l.650-660, 828-834, 872-873): "the kernel never sees the key". #913 had the kernel build, hold and send the Authorization: Basic derived from URL userinfo. On the door path, that now moves to the composition root as a credential of the auth kind.

What

• root::userinfo::UserinfoBasic: a root-owned Basic credential, read at configuration time from a provider base_url, token_url or a service account's token_uri. It decodes the userinfo the way reqwest 0.12.28 did (request.rs:169-182, extract_authority :581-605, util.rs:4-25).
• Door member requests: member_routes wraps the member's auth binding (lead_with). Each answer gets the Basic value first unless it already names authorization, which is reqwest's replace_headers condition (util.rs:50-77).
  • The kernel's MemberRoute and far_end now carry only the binding handle. MemberRoute.userinfo and the far_end Basic logic are removed.
• Token POST: the root's connection table puts the Basic value first; the endpoint is stripped before any auth plugin sees its settings.
  • 1.5.5 sources: v1.5.5 egress_auth/oauth_client_credentials.rs:119-126 and jwt_bearer.rs:175-181. The assertion's aud stays the raw token_uri.
• TRANSITIONAL, legacy lane plane only: each item below is kept and marked TRANSITIONAL: deleted with the legacy lane plane (row 155 C); inside busbar-llm the marker reads deleted with busbar-llm (row 155 C).
  • kernel UrlUserinfo and its send hook;
  • LaneInput.url_userinfo and LaneInput.token_userinfo;
  • bind_presenting;
  • AuthAxis::serving_presenting;
  • the in-process lane's inserts.

REDs

• crates/busbar/src/root/tests/userinfo.rs:
  • the root reads userinfo exactly as reqwest did;
  • a binding is led by the Basic value unless it names authorization, both answered on the spot and when submitted;
  • a token POST head is led the same way.
• crates/busbar/src/root/tests/door_steps.rs, a_members_base_url_userinfo_reaches_its_attempts_only_through_its_binding: a door member whose base URL contains userinfo has the stripped base on its route. The Basic value reaches it only through the binding, first for the api-key style and displaced for bearer.
• Unchanged and still asserting the wire: the door-path OAuth and jwt tests (stripped dial, Basic-led token POST, raw aud) and the legacy lane's tests.