refactor(routes): move cleanup domain into routes/cleanup/ subpackage (#5658)

Slice 2g of the route-domain reorganization (#4082/#4071). Moves
cleanup_routes.py into routes/cleanup/, leaving a backward-compat
sys.modules shim at the old path. Pure file reorganization, no behavior
change.

The shim uses sys.modules replacement so string-targeted
monkeypatch.setattr("routes.cleanup_routes.*", ...) in
test_cleanup_owner_scope.py reaches the canonical module.

Canonical module imports only from src/ and stdlib (zero internal
routes/ coupling). Zero source-introspection landmines.

Adds tests/test_cleanup_routes_shim.py to pin the sys.modules shim
contract. Verified: compileall clean; targeted tests pass.

routes/cleanup/__init__.py

@@ -0,0 +1,5 @@
+"""Cleanup route domain package (slice 2g, #4082/#4071).
+
+Contains cleanup_routes.py, migrated from the flat routes/ directory.
+Backward-compat shim at routes/cleanup_routes.py re-exports from here.
+"""