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>

docs/CHEATSHEET.md

@@ -38,8 +38,21 @@ make clean      # rm .venv, build artifacts
 ## Client Auth
 
 ```
-PASS <network>:<password>       # select network + authenticate
-PASS <password>                 # use first network
+PASS <password>                 # authenticate (all networks)
+```
+
+## Namespacing
+
+```
+#channel/network                # channel on a specific network
+nick/network                    # foreign nick on a specific network
+own-nick                        # own nicks shown without suffix
+```
+
+```
+/msg #libera/libera hello       # send to #libera on libera network
+/join #test/oftc                # join #test on oftc
+/join #a/libera,#b/oftc         # comma-separated, different networks
 ```
 
 ## Connection States
@@ -107,6 +120,7 @@ src/bouncer/
   cli.py         # argparse
   config.py      # TOML loader
   irc.py         # IRC message parse/format
+  namespace.py   # /network encode/decode for multiplexing
   proxy.py       # SOCKS5 connector (local DNS, multi-IP)
   network.py     # server connection + state machine
   client.py      # client session handler