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>

tests/test_operator_import_termux.py

@@ -0,0 +1,40 @@
+"""Guard the operator's lightweight import surface (docs/termux-operator.md P0).
+
+A phone/Termux operator installs only the operator deps (requests, rich,
+websockets, cryptography, srp) — never the server stack (sanic, pydantic) — and
+`srp` may be unbuildable there (pure-Python fallback). So `python -m
+cmd_chat.operator` must import with srp/sanic/pydantic ALL absent. This is easy
+to regress: any eager `import srp`/sanic on the operator path breaks it. Run the
+check in a subprocess so blocking modules can't pollute the test interpreter.
+"""
+
+import subprocess
+import sys
+
+
+_SNIPPET = """
+import sys
+# Simulate a Termux operator box: no srp C-ext build, no server-only deps.
+for missing in ("srp", "sanic", "sanic_ext", "pydantic"):
+    sys.modules[missing] = None  # 'import X' now raises ImportError
+
+import cmd_chat.operator                       # must not need srp/sanic/pydantic
+assert hasattr(cmd_chat.operator, "OperatorBridge")
+
+import cmd_chat.client.client as c             # client must fall back to the shim
+assert c.srp.__name__.endswith("_srp_pure"), c.srp.__name__
+print("OK")
+"""
+
+
+def test_operator_imports_without_srp_or_server_deps():
+    proc = subprocess.run(
+        [sys.executable, "-c", _SNIPPET],
+        capture_output=True,
+        text=True,
+    )
+    assert proc.returncode == 0, (
+        "operator import failed without srp/sanic/pydantic:\n"
+        f"stdout:\n{proc.stdout}\nstderr:\n{proc.stderr}"
+    )
+    assert proc.stdout.strip().endswith("OK"), proc.stdout