swarm-controller: end a dead replay, clamp stale resume ids, test on a real server

A replay consumer now has a 5s idle heartbeat, and the row stream ends at
the first consumer error instead of skipping it. When the server drops the
consumer, as on a restart, two missed heartbeats end the SSE response and
the browser reconnects with Last-Event-ID. Before, the response stayed
open and delivered nothing.

A Last-Event-ID above the stream's newest sequence now replays as a fresh
reader. The server accepts a start past the end and delivers nothing
until the stream reaches it, so a stale id stalled the replay.

swarm-controller's stream tests run against a real nats-server on
loopback (`test_nats`, binary from HIVE_NATS_SERVER, exported by
checks.nix and devshell.nix). They cover: ensuring the stream twice, an
existing stream with other limits being opened as it is, the replay
arriving before live rows, a reconnect getting every later row exactly
once, and a dropped consumer ending the row stream.

The limits test now compares against the subagent stream's own config.
Two comments that described the agent's terminal route as live-only are
corrected.

nix/checks.nix

@@ -222,6 +222,11 @@ in
     # quietly does nothing is worse than no drift test, because it still
     # shows up green.
     HIVE_RESERVED_NAMES = pkgs.lib.concatStringsSep " " (import ./reserved-names.nix);
+    # swarm-controller's stream and replay tests run against a real
+    # `nats-server` on loopback (`swarm-controller/src/test_nats.rs`), since
+    # what they check is the server's own create and delivery behaviour.
+    # Exported here and in `devshell.nix`; the tests panic when it is unset.
+    HIVE_NATS_SERVER = "${pkgs.nats-server}/bin/nats-server";
   };
 
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