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  thin-wrapper reading is superseded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  gix-protocol call — resolved; rider superseded); backend/transport/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

Cargo.toml

@@ -50,8 +50,13 @@ gix-object = { version = "0.64", optional = true, features = ["sha1"] }
 gix-fsck = { version = "0.25", optional = true, features = ["sha1"] }
 gix-hash = { version = "0.26", features = ["sha1"] }
 gix-packetline = "0.22"
-gix-protocol = "0.65"
-gix-transport = "0.59"
+# Consumer half (ADR-017): gix-protocol with its async client (handshake +
+# ls-refs + fetch machinery) driven by a custom alkcall Transport impl
+# (gix-transport's client::Transport is the transport-agnostic seam);
+# the push client is hand-rolled to ADR-013's shapes (gitoxide has no
+# send-pack client).
+gix-protocol = { version = "0.65", features = ["async-client"] }
+gix-transport = { version = "0.59", features = ["async-client"] }
 tokio = { version = "1", default-features = false, features = ["rt", "sync", "io-util", "macros", "time"] }
 tokio-util = { version = "0.7", features = ["compat"] }
 futures = "0.3"