feat(rdp): RD Gateway transport (experimental)

RDP profiles can go through a Remote Desktop Gateway (MS-TSGU over an
HTTPS WebSocket) with its own username and password, stored like other
credentials (rdp-gateway-password). The gateway stream is bridged to a
loopback socket so the blocking RDP client uses it like any transport.

IronRDP's gateway crate (0.0.1) is vendored with fixes: it used
ironrdp-tls, which accepts any certificate, before sending Basic
credentials — the gateway is now verified against the web PKI; its
asserts on gateway-supplied lengths are errors instead of panics; and
the server port is passed through. Checked against a self-signed
openssl s_server: the connection is refused ('invalid peer certificate')
before any request or credential is sent. Not tested against a real
gateway; Basic-auth gateways only (no NTLM).

rust/vendor/README.md

@@ -7,6 +7,7 @@ IronRDP release includes the fix it carries.
 |---|---|---|
 | `picky` | 7.0.0-rc.25 (crates.io) | `=x.y.z-rc.N` RustCrypto pins relaxed to `^x.y.z-rc.N` (so the stable releases russh uses satisfy them); one unused import removed. |
 | `sspi` | 0.21.3 (crates.io) | Same pin relaxation. |
+| `ironrdp-mstsgu` | 0.0.1 (crates.io) | RD Gateway client. Upstream connects with `ironrdp-tls`, which accepts any certificate, and then sends Basic credentials; the patch verifies the gateway against the web PKI (rustls/ring + webpki-roots). Its `assert!`s and unchecked length subtractions on gateway packets become errors, and the RDP server's port is passed through instead of a fixed 3389. |
 | `ironrdp-session` | 0.11.0 (crates.io) | Upstream fixes not yet released: #1486 "preserve bitmap source stride" (commit `80bb81b344`, applied as-is) and the RDP6 orientation flip from #1524 (`8f1737a287`). Without them xrdp desktops render sheared and upside-down tile by tile. |
 
 Why: IronRDP pins `picky = "=7.0.0-rc.25"`, which pins `curve25519-dalek = "=5.0.0-rc.1"` and