Advertise the lazy `to_asyncio` API

Preserve the existing public wildcard surface while adding the lazy
`to_asyncio` submodule to `__all__` and `dir(tractor)`.

Exercise both APIs in cold interpreters and verify normal package
import still leaves `asyncio` unloaded.

Review: PR #478 (goodboy)
https://github.com/goodboy/tractor/pull/478#pullrequestreview-4922213201

(this patch was generated in some part by `opencode` using
`gpt-5.6-sol` (`openai`))

tests/test_lazy_imports.py

@@ -2,6 +2,9 @@
 Regression tests for the cold package import surface.
 
 '''
+import json
+import subprocess
+import sys
 from typing import (
     Any,
     get_type_hints,
@@ -17,6 +20,54 @@ from tractor.ipc import (
 )
 
 
+def run_cold_import(code: str) -> dict[str, object]:
+    result = subprocess.run(
+        [
+            sys.executable,
+            '-c',
+            code,
+        ],
+        check=True,
+        capture_output=True,
+        text=True,
+    )
+    return json.loads(result.stdout)
+
+
+def test_lazy_to_asyncio_package_api():
+    '''
+    Keep the public lazy submodule discoverable without eagerly
+    importing it.
+
+    Before the lazy conversion, package import side effects exposed
+    `to_asyncio` to `dir()` and wildcard imports. Exercise those APIs
+    in cold interpreters so this test proves normal `import tractor`
+    leaves `asyncio` unloaded, while discovery and wildcard access
+    still advertise and resolve the public submodule.
+
+    '''
+    cold = run_cold_import(
+        'import json, sys, tractor; '
+        'print(json.dumps({'
+        '"advertised": "to_asyncio" in dir(tractor), '
+        '"asyncio_loaded": "asyncio" in sys.modules}))'
+    )
+    assert cold == {
+        'advertised': True,
+        'asyncio_loaded': False,
+    }
+
+    wildcard = run_cold_import(
+        'import json; '
+        'from tractor import *; '
+        'print(json.dumps({'
+        '"module": to_asyncio.__name__}))'
+    )
+    assert wildcard == {
+        'module': 'tractor.to_asyncio',
+    }
+
+
 def test_lazy_annotation_names_resolve():
     '''
     Resolve annotations without importing optional dependencies.

tractor/__init__.py

@@ -76,6 +76,19 @@ from .discovery._registry import (
 # from . import hilevel as hilevel
 
 
+__all__: tuple[str, ...] = tuple(
+    name
+    for name in globals()
+    if not name.startswith('_')
+) + (
+    'to_asyncio',
+)
+
+
+def __dir__() -> list[str]:
+    return sorted(set(globals()) | set(__all__))
+
+
 def __getattr__(name: str):
     '''
     PEP 562 lazy sub-module loading, presently only for