refactor(routes): move vault domain into routes/vault/ subpackage (#5780)

Slice 2k of the route-domain reorganization (#4082/#4071). Moves
vault_routes.py into routes/vault/, leaving a backward-compat
sys.modules shim. Pure file reorganization, no behavior change.

tests/test_vault_routes_shim.py

@@ -0,0 +1,11 @@
+"""Regression test for the vault route shim (slice 2k, #4082/#4071)."""
+
+import importlib
+
+import routes.vault_routes as _shim_vault  # noqa: F401
+
+
+def test_legacy_and_canonical_vault_module_are_same_object():
+    legacy = importlib.import_module("routes.vault_routes")
+    canonical = importlib.import_module("routes.vault.vault_routes")
+    assert legacy is canonical