feat: implement channels protocol + ADR-047 (openable ALPNs are operations)

ADR-047: the unifying decision that  dissolves into
per-ALPN ops (, ) with a
 marker on . Each openable ALPN registers
its own ops with their own , ,
, and the marker. The  field is replaced
by  (Sub/Pub). The generic ops (channel/close,
channel/control, channel/resources/subscribe) stay, keyed by
channel_id. Resolves Gaps A-G from the research findings (Gap B broker
named out-of-scope for alkcall; Gap C relay wrapper is consumer
concern; Gap D connection-owner allocates; Gap E extension trait;
Gap F boolean marker on wire; Gap G ACL/ownership complementary).

ADR-037 amended:  dissolves;  removed; generic
ops stay;  preview dropped from resources/subscribe.

Spec docs updated: channel-operations.md (unified model, opener ledger,
ACL flow), operation-registry.md (channel_open marker, ChannelOpenSpec),
README.md (ADR-047), open-questions.md (OQ-31..38 resolved).

Source changes:
- spec.rs: ChannelOpenSpec struct, channel_open field on OperationSpec,
  with_channel_open builder, 3 tests
- discovery.rs: spec_to_json emits channel_open boolean,
  operation_spec_schema includes channel_open, 2 tests
- from_call.rs: rebuild_spec_for parses channel_open marker,
  derive_alpn_from_op_name helper, 6 tests

Channels module (src/channels/, 10 files, ~2400 lines):
- wire.rs: 8-byte chunk header (ChunkHeader, parse/write_header,
  read_header/write_chunk/write_eof async helpers), 12 tests
- reassembly.rs: MpscRecvStream (tokio::mpsc::Receiver<Bytes> →
  AsyncRead), MpscSendStream (AsyncWrite → tokio::mpsc::Sender<Bytes>),
  REQ-CH-01 shutdown sentinel, REQ-CH-02 sender-drop EOF, 10 tests
- mux.rs: MuxHandle (clone-able, register(channel_id)),
  MuxRunner (per-channel pump tasks, exits when handles drop),
  OpenerLedger (ADR-047 §7), 4 tests
- manager.rs: ChannelManager (channel map, open_channel,
  install_channel_zero, route_payload, teardown_channel, clear_all),
  11 tests
- source.rs: ChannelBidiStreamSource (yield-once accept_bi),
  channel_source helper, 4 tests
- adapter.rs: ChannelsAdapter (ProtocolHandler for alknet/channels,
  demux loop, install_channel_zero hook), 1 test
- operations.rs: ChannelOperations (registers channel/close,
  channel/control, channel/resources/subscribe), ChannelCore
  (check_open/on_close wrappers), 4 tests
- policy.rs: ChannelLifecyclePolicy trait, NoCap, PerIdentityChannelPolicy
  (default 256, per_identity_caps override), default_policy, 8 tests
- env.rs: ChannelOperationEnv extension trait (ADR-047 §4),
  ChannelsSessionEnv impl, 2 tests
- client.rs: ChannelClient (from_connection, call_open_op,
  take_call_connection), 1 test

Verification: 432 tests pass (66 new channels + 10 marker + 356
existing), clippy clean, fmt clean, cargo doc generates.

Cargo.toml: +bytes dependency.

docs/architecture/open-questions.md

@@ -41,7 +41,7 @@ status, priority, and (when resolved) a resolution citing the ADR.
 
 | OQ | Title | Status | Priority | Resolution |
 |----|-------|--------|----------|------------|
-| OQ-22 | Call protocol pub/sub primitive — pub to go with sub | partially resolved | high | ADR-046 resolves the primitive: `OperationType::Pub` + `HandlerKind::Sink` + `call.published` wire event + `invoke_sink()` dispatch path. The fan-out/broker mechanism (one producer, N consumers, topic matching) is deferred to the channels session — the call protocol is point-to-point; the broker is a routing concern that sits above it. See §"Pub/Sub Gap" below. |
+| OQ-22 | Call protocol pub/sub primitive — pub to go with sub | partially resolved | high | ADR-046 resolves the primitive: `OperationType::Pub` + `HandlerKind::Sink` + `call.published` wire event + `invoke_sink()` dispatch path. The fan-out/broker mechanism (one producer, N consumers, topic matching) is deferred to the channels session — the call protocol is point-to-point; the broker is a routing concern that sits above it. ADR-047 §1 names the broker (Gap B) as out-of-scope for alkcall; the hub composes it on top of the `Pub`/`Sub` primitives. See §"Pub/Sub Gap" below. |
 
 ### Pub/Sub Gap
 
@@ -74,6 +74,14 @@ is the load-bearing piece the broker composes on.
 |----|-------|--------|----------|------------|
 | OQ-23 | Full channel-level flow-control windowing | deferred(scope) | low | Bounded-buffer decided (ADR-040); full windowing blocked on HOL-blocking deployment observation |
 | OQ-24 | Channels add/strip API shape | open | low | Whether the 8-byte header add/strip is built into the read/write path or a standalone utility. The contract (ADR-035) is decided; the function surface is not |
+| OQ-31 | `channel/open` ACL granularity | resolved | high | ADR-047 — `channel/open` dissolves into per-ALPN ops; each op has its own `access_control` |
+| OQ-32 | Quota lifecycle (opener vs closer, transport drop) | resolved | high | ADR-047 §7 — the per-connection opener ledger; decrement on every teardown path, keyed by opener |
+| OQ-33 | Per-identity connection cap (endpoint layer) | deferred(scope) | low | Named as a separate layer (ADR-047 §"ALPN category reframe" references the findings); belongs at `alknet-endpoint`, not channels. Named to stop the re-tangle |
+| OQ-34 | `channel_open` marker wire format | resolved | medium | ADR-047 §2 — boolean `"channel_open": true` in `services/schema`; ALPN derivable from op name |
+| OQ-35 | `OperationEnv::channel_manager()` coupling | resolved | high | ADR-047 §4 — extension trait `ChannelOperationEnv` in `channels-call`; call crate stays free of channels types |
+| OQ-36 | `channel_id` allocation in Pub case | resolved | medium | ADR-047 §5 — "connection owner allocates" (the side that holds the `ChannelManager`); amends "responder allocates" |
+| OQ-37 | `from_call` relay wrapper for marked ops | open | medium | ADR-047 §1 names it as a consumer (hub) concern; alkcall's `from_call` reconstructs the marker (Gap F resolved) so the consumer can branch on it |
+| OQ-38 | ALPN→path-segment mapping | resolved | low | ADR-047 §"Negative" — strip the `alknet/` prefix; ALPNs without that prefix use the full ALPN string (rare, two-way-door) |
 
 ## Core Types