docs(architecture): ADR-016 — native session preamble (review 001 A-3)

- new ADR-016: channels open-op params pinned as {repo, service}
  (channels/git/sub, additionalProperties: false); direct-ALPN GitAdapter
  parses the git-daemon request line (POC-1-verbatim grammar, capture-
  backed); session tuple gains the service dimension on both substrates;
  GitSession mirrors the shapes; version deliberately stays out of the
  preamble (service fully determines the state machine)
- amend ADR-002/005/010 (tuple, substrate inputs, open-op params pin) and
  transport.md/doors.md/overview.md/backend.md accordingly
- add the ADR-016 wire shapes to OQ-03's freeze inventory; note in
  AGENTS.md convention 9 that the alkgit-specific framing now exists and
  is pinned
- review 001: A-3 marked resolved

verification: cargo test, clippy -D warnings, fmt --check, doc — clean

docs/architecture/transport.md

@@ -22,16 +22,25 @@ full decision (what each substrate owns and why); the surface is:
 
 - **Duplex session** (the `alk/git` ALPN producer, channels-opened git
   sessions, embedder stream doors) — input: (peer identity, resolved repo
-  id, authorized-repo marker, duplex stream, `Limits`). The marker is a
-  type the door/adapter constructs only after the ADR-007 check passes
-  (skipping the check is a type error — ADR-007; D-3). Encapsulates the
-  split/compat/packetline
-  bridge, the request reader (delim-aware parsing, `reset()` discipline,
-  break-on-error), and the sideband writer.
+  id, service, authorized-repo marker, duplex stream, `Limits`). The
+  marker is a type the door/adapter constructs only after the ADR-007
+  check passes (skipping the check is a type error — ADR-007; D-3). The
+  service is selected at establishment on every native path (ADR-016:
+  the channels open-op params `{repo, service}`, or the in-band
+  git-daemon request line the direct-ALPN `GitAdapter` parses,
+  POC-1-verbatim), and selects the state machine: the V2 capability
+  advertisement for fetch, the V0 ref advertisement for push — both are
+  server-emitted firsts, so the service must precede the stream.
+  Encapsulates the split/compat/packetline bridge, the request reader
+  (delim-aware parsing, `reset()` discipline, break-on-error), and the
+  sideband writer.
 - **Stateless session** (smart-http doors, e.g. alkhttp's future `git`
-  feature) — input: (peer identity, resolved repo id, authorized-repo
-  marker, request-reader, response-writer, `Limits`) per http POST; adds
-  the http-framing rules
+  feature) — input: (peer identity, resolved repo id, service,
+  authorized-repo marker, request-reader, response-writer, `Limits`) per
+  http POST. The door's route still selects the service (doors.md), but
+  the substrate input carries it explicitly — nothing in a POST body
+  distinguishes an upload-pack POST from a receive-pack POST before
+  parsing (ADR-016). Adds the http-framing rules
   (capability-dump skip, flush-only responses, probe handling). IO-abstract:
   the door supplies reader/writer; see [doors.md](doors.md) for the
   mounting.
@@ -145,7 +154,8 @@ a partial ref list is the worst failure mode an advertisement can have
 
 Crate-root re-exports (alktty pattern; the full list in
 [backend.md](backend.md) §public API): `GitAdapter` + `register_openable`,
-`GitSession`, substrate types, `Limits`, the backend traits (ADR-010's
+`GitSession`, substrate types (including the service selector and the
+authorized-repo marker), `Limits`, the backend traits (ADR-010's
 seam), protocol error enums; gix-feature types under the feature.
 Publish-freeze point: **OQ-03**.
 
@@ -161,11 +171,13 @@ Publish-freeze point: **OQ-03**.
 | [010](decisions/010-pure-protocol-crate.md) | Pure protocol crate | wire layer is backend-trait-only |
 | [013](decisions/013-receive-pack-state-machine.md) | receive-pack | V0-framed push machine, thin-pack acceptance, unpack-first CAS |
 | [014](decisions/014-v2-negotiation-ack-loop.md) | Negotiation | ack loop, no `ready`, wait-for-done stays |
+| [016](decisions/016-native-session-preamble.md) | Native session preamble | `{repo, service}` open-op params, request-line preamble, service in the tuple |
 
 ## Open Questions
 
 - **OQ-03**: publish/API freeze (partially resolved — single-crate shape
-  settled by ADR-010).
+  settled by ADR-010; the native-path preamble shapes entered the freeze
+  inventory via ADR-016).
 - **OQ-05**: sha256 policy (deferred(scope)).
 - OQ-02 resolved (ADR-014 — ack loop design).
 - OQ-04 resolved (ADR-013 — receive-pack state machine).
@@ -176,7 +188,7 @@ Publish-freeze point: **OQ-03**.
   `docs/research/poc3-findings.md` (the normative wire behavior —
   observed against real git, not docs' grammar)
 - `docs/research/push-captures.md` (the push-path normative record —
-  ADR-013's basis)
+  ADR-013's basis; the git:// request-line framing ADR-016 adopts)
 - `docs/research/negotiation-captures.md` (the negotiation normative
   record — ADR-014's basis)
 - `docs/research/git-protocol.md` (inventory + observed corrections)