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>

config/bouncer.example.toml

@@ -11,11 +11,16 @@ replay_on_connect = true
 host = "127.0.0.1"
 port = 1080
 
+# Client PASS is just the password (no network prefix).
+# A single client connection sees all networks via /network suffixes:
+#   #libera/libera, #debian/oftc, user123/libera
+#
 # Registration uses a random nick and generic ident/realname.
 # After surviving the probation window (no k-line), the bouncer
 # derives a stable nick from the exit endpoint hostname. The same
 # exit IP always produces the same nick across reconnects.
 # Set nick to override (optional, used as fallback only).
+# Network names must not contain '/' (reserved for namespace separator).
 
 [networks.libera]
 host = "irc.libera.chat"