style: satisfy the widened ruff default rule set

ruff 0.16 enables rules that earlier releases did not, so `ruff check .`
was already failing on HEAD with 35 findings - the CI job installs ruff
unpinned and would have gone red on the next push regardless of this
branch.

All changes are mechanical and behaviour-neutral:

- Optional[X] -> X | None, List[str] -> list[str], dropped the now unused
  typing imports (every module already carries
  `from __future__ import annotations`, and requires-python is >= 3.10).
- Sorted import blocks, `import automtu.x as x` -> `from automtu import x`.
- subprocess.run in pmtu.py and wg.py now passes check=False explicitly.
  That is the parameter default, so the calls behave exactly as before.
- int(round(x)) -> round(x): round() with a single argument already
  returns int.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/unit/test_docker.py

@@ -1,7 +1,7 @@
 import unittest
 from unittest.mock import patch
 
-import automtu.docker as docker
+from automtu import docker
 
 
 class TestDocker(unittest.TestCase):

tests/unit/test_output.py

@@ -1,9 +1,9 @@
 import io
 import json
 import unittest
-from contextlib import redirect_stdout, redirect_stderr
+from contextlib import redirect_stderr, redirect_stdout
 
-from automtu.output import OutputMode, emit_json, emit_single_number, Logger
+from automtu.output import Logger, OutputMode, emit_json, emit_single_number
 
 
 class TestOutput(unittest.TestCase):

tests/unit/test_persist.py

@@ -4,7 +4,7 @@ from contextlib import redirect_stdout
 from pathlib import Path
 from unittest.mock import patch
 
-import automtu.persist as persist
+from automtu import persist
 
 
 class TestPersist(unittest.TestCase):

tests/unit/test_pmtu.py

@@ -1,7 +1,7 @@
 import unittest
 from unittest.mock import patch
 
-import automtu.pmtu as pmtu
+from automtu import pmtu
 
 
 class TestPmtu(unittest.TestCase):

tests/unit/test_wg.py

@@ -1,7 +1,7 @@
 import unittest
 from unittest.mock import patch
 
-import automtu.wg as wg
+from automtu import wg
 
 
 class TestWg(unittest.TestCase):