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_extension_registry.py

@@ -12,4 +12,5 @@ def test_load_vm_packer_extension() -> None:
 def test_load_all_includes_vm_packer() -> None:
     extensions = load_all_extensions()
     assert "ext.compose-ssh" in extensions
-    assert "ext.vm-packer" in extensions
+    assert "ext.vm-packer" in extensions
+    assert "ext.saas-stub" in extensions