feat: lift the client's concurrency ceiling from ~15 peers to 1024

The client had never been driven at scale. A stress harness that mixes
searches, downloads, uploads and browses against a local soulfind found four
ceilings, three of which lose work silently rather than merely being slow.

Peers were capped at one per core. ActorSystem ran each actor's whole lifetime
as one job on a fixed-size ThreadPool, and actor loops block until stopped, so
the pool acted as a semaphore: past the cap an actor never started, never
connected and never errored, and uploads stopped working entirely. The pool had
exactly one consumer and every job it received was non-terminating, so it is
gone; actors get their own thread.

TransferResponse raced the bookkeeping it depends on. It was sent straight back
to the peer while UpdateDownloadTokens was still queued for the client loop —
but the response is what invites the peer's file connection, and that connection
is matched by token. Under load the client loop lost, the file connection
matched nothing, and the download sat on Queued forever. The response now goes
out after the token is recorded.

Download tokens were md5(filename) truncated to 20 bits, and the store deletes
every entry matching a token, so two downloads sharing one destroyed each other
— which fetching the same filename from two peers does every time. They now
come from a counter.

Port 0 from GetPeerAddress means the server does not know where a user listens.
It was cached and dialled, and the upload waiting on it was dropped. It is no
longer cached or served against; the connect attempt stays, because its failure
is what makes an unreachable peer fall back to server brokering.

Mixed profile at 1024 peers: 512/512 downloads, 256/256 uploads, 400-850 MiB/s
in ~7s, against 71/96 downloads at 7.9 MiB/s and a 60s timeout before.

benchmarks/stress-baseline.json

@@ -0,0 +1,23 @@
+{
+  "score": 97.39,
+  "parts": {
+    "downloads": 1.0000,
+    "uploads": 1.0000,
+    "searches": 1.0000,
+    "browses": 1.0000,
+    "throughput": 0.8257
+  },
+  "load": {
+    "peers": 1024,
+    "searches": 128,
+    "downloads": 512,
+    "leechers": 256,
+    "browses": 128,
+    "file_bytes": 4194304
+  },
+  "observed": {
+    "downloads_completed": 512,
+    "uploads_completed": 256,
+    "bytes": 3221225472
+  }
+}