Let a host marked for the relay actually be reached through it

The relay checkbox said it was not wired up, and the plan said the server
half was built and shipped. It was not: there was ADR 0004, the contract
records, RelayOptions and a CHECK constraint, and RelayOptions said in its
own remarks that there was no endpoint and no dialler. So both halves are
built here, as step 3 of docs/reaching-a-host-you-cannot-dial.md.

Server, in Features/Relay:

- POST /api/v1/relay/tickets makes every decision: the host, Connect on
  its vault, the relay switched on, and the stored name resolved once and
  judged by RelayAddressPolicy. Loopback, link-local, 0.0.0.0/8,
  multicast and the metadata endpoints are refused with no configuration
  escape, with IPv4-mapped and NAT64 spellings unwrapped first; then the
  operator's denials, allow-list and private-network switch. What passes
  goes into a Data Protection ticket, and nothing resolves the name again.
  Every target refusal is the same 403, so the endpoint cannot be used to
  map the operator's DNS; the real reason is logged.
- GET /api/v1/relay/connect takes the ticket as a second subprotocol,
  checks its expiry against TimeProvider and spends it with a unique
  insert into relay_ticket_use before accepting the upgrade. After the
  upgrade a refusal is a close code: 4502 for a host the server could not
  reach, 4429 for a limit, 4408 idle, 4409 lifetime, 1001 after a drain.
- relay_session is the audit row, written from short-lived scopes.

Client:

- RelayBridge is a loopback SOCKS5 listener that accepts once and pipes
  to the relay socket. It rides on the new SshLoopbackProxy.Lifetime, so
  the connection disposes it on every way a session ends and a failed
  connect disposes it before throwing.
- ServerRelay, exposed as IVaultServer.Relay from /api/v1/meta, asks for
  the ticket, opens the socket, starts the bridge and turns every refusal
  into a sentence.
- VaultViewModel and TransfersViewModel reach a relayed host through it,
  never fall back to dialling it directly, and replace SSH.NET's transport
  error with the relay's own reason when it gave one.

Neither side ever cancels a WebSocket operation. Cancelling one aborts
the socket, and an aborted socket sends no close frame, which is the only
way the client learns the server could not reach the host.
AnIdleSession_IsClosedWithTheIdleCode asserts that the code arrives.

Three departures from ADR 0004, recorded in it: awaited copy loops rather
than Pipelines (same backpressure, two 32 KiB buffers), no ?ticket=
fallback, and no device id in the ticket.

Tests: API 245, shell 582 (5 skipped), layout 170, SSH 110 including a
real login through the bridge, API client 30, contracts 25. Not run: the
Android build (its change is copy only) and the Alpine layout oracle.

src/DodoSSH.Client.Android/Views/HostsScreen.axaml

@@ -1199,19 +1199,18 @@
             port into plaintext columns the server can read, which is the single deliberate concession in
             the design — see ADR 0004 — so it says so rather than being a switch labelled "relay".
 
-            ◆ AND IT SAYS, FIRST, THAT IT BUYS NOTHING TODAY. The relay's server half is built and this
-            client has no path to it — the connect path dials the address directly whether this is ticked or
-            not — so the box as it stood spent that concession and delivered nothing for it. Left tickable
-            rather than disabled, so a host already carrying the flag can lose it. See
+            It also says the two things a user would otherwise find out by failing: the server, not this
+            phone, has to be able to reach the address, and a relayed host does not fall back to dialling
+            directly, so it connects only while the server is reachable. See
             docs/reaching-a-host-you-cannot-dial.md.
           -->
           <CheckBox IsChecked="{Binding EditorRelayEnabled}" MinHeight="44">
             <TextBlock Classes="mono" FontSize="11.5"
-                       Text="Reach this host through the server relay (not wired up yet)"
+                       Text="Reach this host through the server relay"
                        TextWrapping="Wrap" />
           </CheckBox>
           <TextBlock Classes="body"
-                     Text="Not built yet: this app always dials the host itself, so ticking this stores the address on the server and changes nothing about how the host is reached. When it works, the relay will dial on your behalf — which is why the address and port have to be stored in the clear. Everything else about the host stays encrypted either way, and a relayed host needs a port of its own rather than its group's." />
+                     Text="The server dials this host on your behalf, for a machine this phone cannot reach — so the server has to be able to reach it. That is why the address and port are stored on the server in the clear; everything else about the host stays encrypted. A relayed host needs a port of its own rather than its group's, and connects only while you are online." />
 
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