fix(operator): don't import the server stack on the operator path

cmd_chat/__init__.py eagerly imported run_server (sanic/pydantic) and Client,
so any `cmd_chat.*` import — including `python -m cmd_chat.operator` — pulled
the server stack at package init. On a phone/Termux operator that installs only
the operator deps (and where srp may be unbuildable), this failed before the
client's pure-SRP fallback could help.

Make both imports lazy inside main()'s serve/connect branches. The operator
path now imports cleanly with srp/sanic/pydantic all absent (client falls back
to _srp_pure). Add a subprocess regression test to keep it that way.

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

cmd_chat/__init__.py

@@ -2,8 +2,11 @@ import argparse
 import getpass
 import os
 
-from cmd_chat.server.server import run_server
-from cmd_chat.client.client import Client
+# NOTE: the server (run_server) and client (Client) are imported lazily inside
+# main() below. Importing them here would pull sanic/pydantic (server) at
+# package-init time for *any* `cmd_chat.*` import — including
+# `python -m cmd_chat.operator` on a phone/Termux where those server-only deps
+# aren't installed. See docs/termux-operator.md (Phase 0).
 
 
 def resolve_password(args_password: str | None, prompt: str = "Room password: ") -> str:
@@ -43,6 +46,8 @@ def main():
     args = parser.parse_args()
 
     if args.command == "serve":
+        from cmd_chat.server.server import run_server
+
         password = resolve_password(args.password)
         run_server(
             host=args.ip_address,
@@ -53,6 +58,8 @@ def main():
             no_tls=args.no_tls,
         )
     elif args.command == "connect":
+        from cmd_chat.client.client import Client
+
         password = resolve_password(args.password)
         Client(
             server=args.ip_address,