fix(launcher): remove orphaned client-in-game helpers

A launcher can exit while its long-lived close/hide helper remains. The next
run then starts another watcher, so several copies watch the same prefix.

Add an opt-in RCLOL_CLEAN_STALE_HELPERS=1 preflight that removes only helper
processes of this prefix reparented to init/systemd. Helpers of a live
launcher, other prefixes, the game, Riot/League/CEF, darlingserver and the
runtime are preserved. Unreadable or relative process data leaves a process
alone.

Validated across repeated launcher starts: the cleanup removed four, then one,
then one orphaned watcher; a subsequent dry run reported no orphaned helpers.
Zombie reaping after partial recovery is tracked separately in #124.

tests/test_clean_stale_helpers.py

@@ -0,0 +1,110 @@
+"""Exercise orphan-helper cleanup with fake /proc entries and mocked signals."""
+import importlib.util
+import io
+import os
+from pathlib import Path
+import signal
+import tempfile
+import unittest
+
+ROOT = Path(__file__).resolve().parents[1]
+spec = importlib.util.spec_from_file_location(
+    'clean_stale_helpers', ROOT / 'tools/clean-stale-helpers.py')
+clean_stale_helpers = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(clean_stale_helpers)
+
+HELPER = '/repo/tools/close-league-client-in-game.py'
+HIDE_HELPER = '/repo/tools/hide-league-client-in-game.py'
+
+
+class CleanStaleHelpersTests(unittest.TestCase):
+    def setUp(self):
+        self.temp = tempfile.TemporaryDirectory()
+        self.addCleanup(self.temp.cleanup)
+        self.root = Path(self.temp.name)
+        self.proc = self.root / 'proc'
+        self.proc.mkdir()
+        self.prefix = self.root / '.darling'
+        self.other = self.root / 'other-prefix'
+
+    def parent(self, pid, comm):
+        folder = self.proc / str(pid)
+        folder.mkdir(exist_ok=True)
+        (folder / 'comm').write_text(comm + '\n')
+
+    def process(self, pid, script=HELPER, parent_pid=500, parent_comm='systemd',
+                prefix='own', argv0='/usr/bin/python3', state='S',
+                environment=True, arguments=('--detach',)):
+        folder = self.proc / str(pid)
+        folder.mkdir()
+        argv = [argv0, script, *arguments]
+        (folder / 'cmdline').write_bytes(
+            b'\0'.join(arg.encode() for arg in argv) + b'\0')
+        if parent_comm is not None:
+            self.parent(parent_pid, parent_comm)
+        fields = [state, str(parent_pid)] + ['0'] * 17 + ['100']
+        (folder / 'stat').write_text(f'{pid} (python3) ' + ' '.join(fields) + '\n')
+        if environment:
+            prefix = self.prefix if prefix == 'own' else prefix
+            (folder / 'environ').write_bytes(
+                b'DPREFIX=' + os.fsencode(prefix) + b'\0')
+        else:
+            (folder / 'environ').write_bytes(b'UNRELATED=1\0')
+        return folder
+
+    def run_clean(self, dry_run=False):
+        killed = []
+
+        def kill(pid, signum):
+            killed.append((pid, signum))
+
+        out = io.StringIO()
+        result = clean_stale_helpers.clean(
+            self.prefix, proc_root=self.proc, kill=kill,
+            sleep=lambda _seconds: None, out=out, dry_run=dry_run)
+        return result, killed, out.getvalue()
+
+    def test_removes_only_orphaned_helpers_of_this_prefix(self):
+        self.process(100, HELPER)
+        self.process(101, HIDE_HELPER)
+        self.process(200, HELPER, parent_pid=600, parent_comm='darling')
+        self.process(201, HELPER, prefix=self.other)
+        self.process(202, HELPER, argv0='/usr/bin/vim')
+        self.process(203, HELPER, state='Z')
+        self.process(204, HELPER, environment=False)
+        self.process(205, HELPER, parent_pid=700, parent_comm=None)
+        result, killed, output = self.run_clean()
+        self.assertEqual(result, 0)
+        self.assertEqual(sorted(killed), sorted([
+            (100, signal.SIGTERM), (100, signal.SIGKILL),
+            (101, signal.SIGTERM), (101, signal.SIGKILL),
+        ]))
+        self.assertIn('removing 2 orphaned helper(s): 100,101', output)
+
+    def test_dry_run_reports_without_signalling(self):
+        self.process(100)
+        result, killed, output = self.run_clean(dry_run=True)
+        self.assertEqual((result, killed), (0, []))
+        self.assertIn('would remove 1 orphaned helper(s): 100', output)
+
+    def test_no_orphans_is_a_noop(self):
+        self.process(200, parent_pid=600, parent_comm='darling')
+        result, killed, output = self.run_clean()
+        self.assertEqual((result, killed), (0, []))
+        self.assertIn('no orphaned helpers', output)
+
+    def test_relative_helper_prefix_is_preserved(self):
+        self.process(100, prefix='relative-prefix')
+        result, killed, output = self.run_clean()
+        self.assertEqual((result, killed), (0, []))
+        self.assertIn('no orphaned helpers', output)
+
+    def test_launcher_runs_cleanup_before_starting_a_helper(self):
+        source = (ROOT / 'tools/run-riot-client.sh').read_text()
+        self.assertIn('RCLOL_CLEAN_STALE_HELPERS', source)
+        self.assertLess(source.index('clean-stale-helpers.py'),
+                        source.index('close-league-client-in-game.py'))
+
+
+if __name__ == '__main__':
+    unittest.main()