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.

src/channels/mod.rs

@@ -0,0 +1,44 @@
+//! Channels protocol: N logical channels over one transport stream
+//! (ADR-034, amended by ADR-035 — no `stream_type` concept).
+//!
+//! Channel 0 is pre-negotiated as `alknet/call` (ADR-036); channels
+//! 1..N are opened dynamically via per-ALPN open ops on channel 0
+//! (ADR-047). The channels layer is a re-framing proxy: it converts
+//! between "one transport stream carrying N channels" (the wire) and
+//! "N independent `BiStream` handles" (what handlers see).
+//!
+//! ## Module layout
+//!
+//! - [`wire`]: the 8-byte chunk header (sync core, WASM-clean).
+//! - [`reassembly`]: per-channel `MpscSendStream` / `MpscRecvStream` —
+//!   the reassembled `AsyncRead + AsyncWrite` pair the handler sees.
+//! - [`mux`]: the mux (`MuxHandle` / `MuxRunner`) — frames per-channel
+//!   bytes back onto the transport.
+//! - [`manager`]: `ChannelManager` + `ChannelState` + the channel map.
+//! - [`source`]: `ChannelBidiStreamSource` — implements
+//!   `BidiStreamSource` for a single channel (yield-once `accept_bi`).
+//! - [`adapter`]: `ChannelsAdapter` — `ProtocolHandler` for
+//!   `alknet/channels` (the demux loop).
+//! - [`operations`]: `ChannelOperations` — registers `channel/close`,
+//!   `channel/control`, `channel/resources/subscribe` on the call
+//!   `OperationRegistry`; the per-ALPN open ops are registered by the
+//!   ALPN crates via `ChannelCore` (ADR-047 §3).
+//! - [`policy`]: `ChannelLifecyclePolicy` + `PerIdentityChannelPolicy`
+//!   (ADR-041, amended by ADR-047 §7 — opener ledger).
+//! - [`client`]: `ChannelClient` — transport-agnostic
+//!   `from_connection` (ADR-043).
+//! - [`env`]: `ChannelOperationEnv` extension trait (ADR-047 §4 —
+//!   keeps the call crate free of channels types).
+//!
+//! See `docs/architecture/` for the full specification.
+
+pub mod adapter;
+pub mod client;
+pub mod env;
+pub mod manager;
+pub mod mux;
+pub mod operations;
+pub mod policy;
+pub mod reassembly;
+pub mod source;
+pub mod wire;