docs(architecture): ADR-017 — consumer half, GitSession as typed client (review 001 A-5)

- new ADR-017: GitSession is a real typed client in v1 (ls_refs/fetch/
  push), grounded in the two deployment use cases — the p2p replicator
  is the named downstream and needs the client protocol layer; the
  thin-wrapper reading is superseded
- fetch client reuses gix-protocol (async-client) over a custom
  alkcall gix_transport::client::Transport impl (handshake writes the
  ADR-016 request line on the direct path; the channels open-op params
  carry it otherwise); push hand-rolled to ADR-013's shapes (gitoxide
  has no send-pack client)
- storage-agnostic: packs stream both ways to caller-owned consumers;
  no in-session credentials (alkcall transport authenticates); client
  sessions carry ADR-009 Limits (client is also internet-facing)
- manifest: gix-protocol/gix-transport gain async-client features
  (verified against published tree, MSRV 1.88)
- amend ADR-010 (consumer-half bullet) and ADR-012 §4 (the deferred
  gix-protocol call — resolved; rider superseded); backend/transport/
  overview/doors wording; review 001 A-5 marked resolved

verification: cargo check (default, --all-features, --no-default-features,
--features sha256), cargo +1.88 check, cargo test, clippy
-D warnings, fmt --check — clean across the matrix

docs/architecture/backend.md

@@ -130,8 +130,8 @@ Crate-root re-exports (the alktty pattern): backend traits + types
 function, the authorized-repo marker type (the door constructs it only
 after the ADR-007 check passes — session tuples carry it, transport.md
 substrate layer), and the `git/repo/*` op spec+handler pairs),
-`GitAdapter`/
-`register_openable` (producer), `GitSession` (consumer), substrate
+`GitAdapter`/`register_openable` (producer), `GitSession` (consumer —
+the ADR-017 typed client: `ls_refs`/`fetch`/`push`), substrate
 types, `Limits`, protocol error enums; feature types (`GixBackend`
 family under `gix`, the file registry under `registry-file`) exported
 under their features. The embedder-facing freeze point remains OQ-03