feat: multi-network namespace multiplexing

Multiplex all networks onto a single client connection using /network
suffixes on channels and nicks. PASS is now just the password (no
network prefix). Channels appear as #channel/network, foreign nicks as
nick/network, own nicks stay bare.

New namespace.py module with pure encode/decode functions. Router
tracks clients globally (not per-network), namespaces messages before
delivery. Client attaches to all networks on connect, sends synthetic
JOIN/TOPIC/NAMES for every channel across all networks.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_config.py

@@ -97,6 +97,19 @@ password = "x"
         with pytest.raises(ValueError, match="at least one network"):
             load(_write_config(config))
 
+    def test_slash_in_network_name_raises(self):
+        config = """\
+[bouncer]
+password = "x"
+
+[proxy]
+
+[networks."lib/era"]
+host = "irc.example.com"
+"""
+        with pytest.raises(ValueError, match="must not contain '/'"):
+            load(_write_config(config))
+
     def test_tls_default_port(self):
         config = """\
 [bouncer]