[M5] mxaccess: F26 step 1 — AsbTransport bridges AsbClient into Transport trait

First slice of F26. Bridges F25's working AsbClient into the M0
`mxaccess::Transport` trait that Session uses to discriminate
operations across NMX and ASB transports.

API additions:
* `mxaccess::AsbTransport<T>` — generic over the same
  AsyncRead+AsyncWrite+Unpin+Send+Sync+'static bound that AsbClient
  takes. Owns an AsbClient and exposes it via `client_mut()` /
  `into_client()`.
* `impl Transport for AsbTransport<T>`:
  - `capabilities()` — `buffered_subscribe = false`,
    `activate_suspend = false`, `operation_complete_frame = false`
    per `design/60-roadmap.md` M5 (no NMX-specific extensions on
    ASB).
  - `kind()` — `TransportKind::Asb`.

Path-dep wiring: `mxaccess` now imports `mxaccess-asb` +
`mxaccess-asb-nettcp` directly.

Compile-time `Send + Sync + 'static` assertion guards the
trait-bound contract.

2 new tests:
* `asb_transport_kind_is_asb`.
* `asb_transport_capabilities_disable_buffered_and_activate_suspend`.

Stubbed for F26 step 2:
* `Session::connect_asb` constructor that owns TCP open +
  preamble + DH handshake orchestration.
* Operation routing that maps ASB types (ItemStatus, RuntimeValue)
  back to mxaccess types (MxStatus, DataChange, MxValue).

Stubbed for F26 step 3:
* Subscription routing — Session::subscribe on ASB needs F25
  subscription operations (CreateSubscription / AddMonitoredItems
  / Publish), which are not yet implemented.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

rust/crates/mxaccess/Cargo.toml

@@ -14,6 +14,8 @@ mxaccess-callback = { path = "../mxaccess-callback" }
 mxaccess-galaxy   = { path = "../mxaccess-galaxy" }
 mxaccess-nmx      = { path = "../mxaccess-nmx" }
 mxaccess-rpc      = { path = "../mxaccess-rpc" }
+mxaccess-asb      = { path = "../mxaccess-asb" }
+mxaccess-asb-nettcp = { path = "../mxaccess-asb-nettcp" }
 thiserror         = { workspace = true }
 tokio             = { workspace = true }
 tracing           = { workspace = true }