fix!: make concurrent invocations work instead of silently seeing nothing (#22)

Running several one-shot commands at once returned "no results" for queries that
had plenty. Four defects, each of which made a dead session look like an empty
network:

- The peer listener bound its port inside a detached thread and panicked on
  EADDRINUSE, unwinding that thread alone. The process carried on with no
  listener, so no peer could deliver a search response. `Listen::bind` now takes
  the socket before login and falls back to an ephemeral port when the
  configured one is held.
- SetWaitPort advertised the configured port whether or not it was bound, so
  peers dialled a sibling process that discarded responses to tickets it never
  issued. The port the listener really holds is what gets advertised, mapped
  through UPnP, and reported by `whoami` and `serve`.
- A zero-byte read is EOF on a TCP socket, but MessageReader reported it as
  success, so a server that closed cleanly left the actor polling a dead socket.
  It is now an error, and Relogged (code 41) has a handler. The peer actor
  classifies that error: a control connection that was established and then
  closed is a clean disconnect, because peers drop idle sockets while the bytes
  move over their own connection, and reporting an error there fails every
  download queued with that peer including the one streaming. A connection that
  never established keeps the error path, which is what the server-brokered
  fallback reads to know a peer is unreachable.
- A session the server cut off - usually because another process logged in under
  the same name - exited 4, documented as "it worked and found nothing". Callers
  concluded the file was unavailable. Sessions now record why they ended and
  those verdicts become exit 7.

The server allows one session per account, so concurrency still needs a distinct
--username per run; SKILL.md documents that. The listener port needs no such
care any more.

Six e2e tests against soulfind cover it, each of which fails without the fix: a
taken listen port, a displaced session, three clients searching at once under
one configured port, the two CLI equivalents, and a peer that hangs up its
control connection while a transfer is still to come.

BREAKING CHANGE: `Listen::start` is replaced by `Listen::bind` and
`Listen::serve`, so the caller owns the socket and can read the port it actually
bound. `Client::listen_port` and `Client::session_loss` are new.

soulseek-rs/src/SKILL.md

@@ -38,6 +38,7 @@ that form; the columns are a subset of the JSON keys.
 | 4 | It worked and found nothing | Widen or correct the query — retrying it verbatim will not help |
 | 5 | Gave up waiting | Raise `--timeout`, or `--search-timeout` for searches |
 | 6 | A transfer started and did not finish | The peer stalled; try another result |
+| 7 | The session ended mid-command (usually another login took the username) | Nothing this run saw is reliable; retry, and give each concurrent run its own `--username` |
 
 ## Commands
 
@@ -148,6 +149,22 @@ soulseek-rs user someuser --json
 `download --stdin` reads whole records, so filtering with `jq` and piping back
 is the normal way to be selective.
 
+## Running several at once
+
+The server allows one session per account, and the later login wins: two runs
+sharing a username silently kill each other's session. Give every concurrent
+run its own name.
+
+```bash
+for q in 'gary beck fold' 'marcal steady'; do
+  soulseek-rs search "$q" --username "$SOULSEEK_USERNAME-$RANDOM" --json &
+done
+wait
+```
+
+The listener port needs no such care: a run that finds the configured port
+taken binds a free one and tells the server about that one instead.
+
 ## Rules
 
 - **Remote paths belong to the peer.** They are backslash-separated and may
@@ -159,6 +176,8 @@ is the normal way to be selective.
   searches. There is no unbounded form.
 - **Exit 4 is not a failure.** A search that finds nothing means the query was
   too narrow or misspelled. Widen it; use `whoami` to rule out the connection.
+- **Exit 7 is not an answer.** The session ended mid-command, so nothing it saw
+  counts. Retry under a name no other run is using.
 - **Never put a password in argv** — `ps` shows it to every user on the box.
   Use `--password-stdin`, the OS keychain, or `SOULSEEK_PASSWORD_CMD`.
 - **Downloading pulls a file off a stranger's machine.** Confirm with the user