refactor(tty): split control channel into STREAM_CTRL_IN/STREAM_CTRL_OUT halves (Phase 7)

The single STREAM_CONTROL = 3 was documented as bidirectional but the
adapter had to ignore Exit from the client because the two directions
were indistinguishable on the same stream_type — half-duplex in
disguise. Phase 7 splits it into two halves so the bidirectionality is
literal on the wire.

Changes:
- wire.rs: STREAM_CTRL_IN = 3 (client→server), STREAM_CTRL_OUT = 4
  (server→client); InvalidStreamType bound > 3 → > 4; Chunk::control
  → Chunk::ctrl_in/ctrl_out; ChunkWriter::write_control_json →
  write_ctrl_in_json/write_ctrl_out_json; tests split accordingly.
- control.rs: ControlMessage doc updated with the stream_type column;
  JSON shape unchanged.
- adapter.rs: pump_client_to_backend dispatches on STREAM_CTRL_IN
  (Resize/Signal/Eof; Exit on ctrl_in is a protocol violation,
  ignored); send_exit_chunk emits on STREAM_CTRL_OUT; STREAM_CTRL_OUT
  from the client is a protocol violation, ignored. 3 new tests for
  the direction enforcement; existing tests updated to the new
  stream_types.
- negotiation.rs: framing-disambiguation doc updated (server-sent
  stream_type set is {1, 2, 4}).
- alknet-tty-local/tests: common/mod.rs, pty.rs, pipe.rs updated to
  the new constants.

Specs:
- ADR-052 amended (§4a 'Control channel split (Phase 7 amendment)').
- tty-wire.md + tty-adapter.md updated (last_updated 2026-07-18).

Verification:
- cargo test -p alknet-tty: 65 passed (was 61; +4 new tests).
- cargo test -p alknet-tty-local: 19 passed.
- cargo test --workspace --all-features: 1017 passed, 0 failed.
- cargo clippy --workspace --all-features: clean.
- cargo fmt --all: clean.

crates/alknet-tty-local/tests/pty.rs

@@ -17,7 +17,7 @@ mod common;
 use std::sync::Arc;
 use std::time::Duration;
 
-use alknet_tty::wire::{STREAM_CONTROL, STREAM_STDIN};
+use alknet_tty::wire::{STREAM_CTRL_OUT, STREAM_STDIN};
 use alknet_tty_local::LocalTtyBackend;
 use common::{negotiate_pty_json, spawn_session};
 
@@ -245,7 +245,7 @@ async fn pty_exit_chunk_is_last() {
         if saw_exit {
             panic!("chunk arrived after exit: stream_type={st}, bytes={bytes:?} (ADR-055)");
         }
-        if st == STREAM_CONTROL {
+        if st == STREAM_CTRL_OUT {
             let v: serde_json::Value = serde_json::from_slice(&bytes).unwrap();
             if v["type"] == "exit" {
                 assert_eq!(v["code"], 0);