fix(ssh): linear host-pattern matching, per-hop forwarding gates, loopback remote forwards

- known_hosts: the @cert-authority pattern matcher was recursive and
  backtracking; a pattern of a few hundred thousand stars overflowed the
  connect thread's stack on every handshake and '*a*a*a…b' went
  exponential. It is now the iterative two-pointer match, and patterns
  over 1 KiB are refused on import and in Trust a CA.
- Agent-forward and remote-forward channels are only honoured on the
  final hop: a jump host could previously open auth-agent channels and
  reach remote-forward destinations on the client machine.
- Remote forwards with no bind address listen on the server's loopback
  (as ssh -R does) instead of every interface.
- Keyboard-interactive gives up after 10 rounds; dials through jump hosts
  and run_command have timeouts; the handshake timeout leaves room for
  the host-key prompt so a slow reader isn't timed out mid-decision.
- Known hosts are looked up per key type, so an imported known_hosts with
  ed25519 and RSA rows no longer produces a false 'key changed' warning.
- SFTP download suggests a sanitised base name in the save dialog.

docs/user-manual.md

@@ -105,7 +105,7 @@ If you connect to a profile that already has a live session, Nexus Terminal asks
 - **Key & Passphrase**: choose a key from the suggestions (keys in `~/.ssh`), click **Browse**, or type a full path (`~` is not expanded). The editor checks that the file exists and is a private key.
 - **Host Verification**: the host key policy (see section 12).
 - **Jump Hosts**: one or more bastions, each with user, host, port and an optional key. Nexus Terminal tunnels through each hop in order.
-- **Port Forwarding**: local and remote forwards, each with a bind address and port, destination, label and an **Active** box to start it on connect.
+- **Port Forwarding**: local and remote forwards, each with a bind address and port, destination, label and an **Active** box to start it on connect. A remote forward with no bind address listens on the server's loopback only (as `ssh -R` does); set it to `0.0.0.0` to expose it to the server's network, which also needs `GatewayPorts` on the server.
 - **Startup**: a startup command and environment variables (`KEY=VALUE`, one per line).
 - **Login Script**: expect-style steps (see below).
 - **SSH Agent**: agent authentication and **Forward Agent to Remote** (like `ssh -A`).