Tell being away from giving up

Leaving was the only way out of a network, and it is the irreversible one:
it publishes a release and then removes the configuration, the secret, the
network's signed records, its cached hints and the protocol key it used.
What somebody usually wants before a reboot, a trip or an experiment is
the other thing — stop serving it and keep everything.

`tsunagi network stop <id>` closes that network's sessions, takes its
address off the interface and keeps it from starting again. Nothing is
announced, deliberately: to the others this device is away, which is an
ordinary condition they already handle, and the address and name it holds
stay reserved for it. `tsunagi network start <id>` resumes it where it left
off. Both are remembered, so a restart does what the last instruction said
rather than what the last command line happened to say.

Except when the command line says otherwise: `up --network X` starts X
whatever its stored state, because a command naming a network is an
instruction to run it. The banner now says which of the three happened —
`new`, `already here`, or `was stopped; this command starts it` — since
silently, that is a stop that comes back from the dead with nothing to
explain it.

The listing tells the three states apart too: running with its address,
stopped and kept, or configured and waiting for an agent to start. Each row
says what to type to move it, because "stop" and "leave" are a pair that
has to be easy to tell apart before the irreversible one is typed.

The local control protocol is 11.

Covered end to end against a running agent: stopping leaves it configured
and says so, stopping twice is the state asked for rather than an error,
starting brings it back, and the secret afterwards is the one from before —
so it is the same network and not a lookalike.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/testing.md

@@ -71,8 +71,9 @@ live one is not, and the derived socket path staying short enough to bind.
 
 `crates/tsunagi-cli/tests/network_cli.rs` runs the real binary too: joining
 with no secret invents one, prints it in full and prints a line the other
-side can paste unchanged, while a bare name this device already knows
-resumes that network instead of inventing another of the same name.
+side can paste unchanged; a bare name this device already knows resumes
+that network instead of inventing another of the same name; and a network
+can be stopped and started again with its secret and its place intact.
 
 `crates/tsunagi-cli/tests/dns_service.rs` runs the real binary: the resolver
 comes up with no interface to attach it to, the listener is not rebuilt on