feat: channels path carries the negotiation in the open op (L1, L3)

The channels path no longer carries a second negotiation frame on the
channel's data stream (ADR-009). The open op's registry-validated
input IS the negotiation:

- producer: make_tty_open_handler parses the open op's input into a
  NegotiateRequest and drives the new drive_session_pre_negotiated
  (same three-pump driver as drive_session, minus the wire-frame
  negotiation phase; validate/allocate factored into
  validate_and_allocate, shared by both paths). Post-open failures
  (unknown backend, allocate_failed, ownership denial) still go to the
  client as a 0x00-prefixed negotiation error frame, so the consumer's
  M1 disambiguation read applies unchanged. The tty:open scope gate is
  enforced by the registry's AccessControl (not re-checked in the
  handler).
- consumer: open_via_channels parses params locally (fail-fast before
  a channel is allocated), opens the channel, and starts raw-chunk
  mode directly (from_halves_raw — no negotiation write; the
  0x00-error-frame peek retained).
- tty_open_spec's input schema is now the partial NegotiateRequest
  shape (carriage/backend/cmd required; backend params stay free-form
  — raw JSON Schema is permissive on unknown keys).

Prerequisites landed upstream: alkcall 0.4.0 enforces
OperationSpec.input_schema at dispatch (the registry check this design
leans on never existed before); alkcall 0.4.1 parks early-arrival
chunks for un-adopted channels instead of dropping them — the open
response / producer's-first-write race was silently losing the first
chunks (found by L3's test; the session never resolved).

L3: open_via_channels + from_bidi_stream_via now covered end-to-end
(5 session tests + pre-negotiated adapter test + shared-harness tests
in the new crate::testing module; the channels harness moved there so
session tests share it).

Verification: cargo test 93 lib (default), 116 lib + 19 integration
(--all-features); clippy -D warnings native + wasm clean; fmt clean;
doc 0 warnings; wasm check clean.

Cargo.lock

@@ -27,9 +27,9 @@ dependencies = [
 
 [[package]]
 name = "alkcall"
-version = "0.3.1"
+version = "0.4.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d4116dba284601e3337e918d4a4256706e43dd8bb6a30095bf96d664ae58d74a"
+checksum = "9badefe048a194c93eed09bc5326ebf092fc86817e561d88de2cd12b6302753a"
 dependencies = [
  "async-trait",
  "bytes",