Implement SAND-WP-0006: SaaS payments, routing, and ext.saas-stub

Add credits store, metering on create/destroy, extension routing resolver,
metered SaaS stub extension, burst/saas profiles, credits CLI, docs, and tests.

tests/test_manager.py

@@ -53,8 +53,9 @@ def test_create_and_destroy(store: SandboxStore) -> None:
         consumer=Consumer(actor=ActorType.ADM, project="sand-boxer"),
     )
 
+    fake = FakeBackend()
     with (
-        patch("sandboxer.core.manager.resolve_backend", return_value=FakeBackend()),
+        patch("sandboxer.core.manager.resolve_backend", return_value=fake),
         patch("sandboxer.core.manager.emit_lifecycle_event", return_value=None),
         patch("sandboxer.core.manager.resolve_host", return_value="coulombcore"),
     ):