perf(pre-commit): batch+parallelize pytest on commit; kill 15s sleeps in steam_backlog_enforcer tests

- meta/.pre-commit-config.yaml: move pytest-coverage hook to pre-commit stage
- scripts/pytest_changed_packages.py: single batched pytest -n auto invocation
  with one --cov flag per affected python_pkg subpackage, wrapped in
  systemd-run --user --scope -p MemoryMax=4G -p MemorySwapMax=0 when available
- python_pkg/steam_backlog_enforcer/tests/conftest.py: new autouse
  _no_real_sleep fixture patches time.sleep across game_install /
  library_hider / steam_api / _enforce_loop. Removes 3x 15s real sleeps
  in TestFinalizeCompletion that fired through _ensure_steam_running

steam_backlog_enforcer test wall time: 33.97s -> 5.61s (xdist, no-cov)
5-package batched run: 732 tests in 1.37s @ 668% CPU
Coverage stays at 100% on all affected packages.

Evidence: docs/superpowers/evidence/pre-commit-pytest-batch-2026-05-14.json

steam_backlog_enforcer/tests/conftest.py

@@ -116,3 +116,21 @@ def _block_real_subprocesses() -> Iterator[None]:
         ),
     ):
         yield
+
+
+@pytest.fixture(autouse=True)
+def _no_real_sleep() -> Iterator[None]:
+    """No-op every ``time.sleep`` used by the package.
+
+    Several modules call ``time.sleep`` for Steam-launch / install-retry /
+    rate-limit pacing.  Individual tests that need to observe sleep
+    behaviour can override these patches inside their own ``with`` block.
+    """
+    noop = MagicMock()
+    with (
+        patch("python_pkg.steam_backlog_enforcer.game_install.time.sleep", noop),
+        patch("python_pkg.steam_backlog_enforcer.library_hider.time.sleep", noop),
+        patch("python_pkg.steam_backlog_enforcer.steam_api.time.sleep", noop),
+        patch("python_pkg.steam_backlog_enforcer._enforce_loop.time.sleep", noop),
+    ):
+        yield