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_saas_stub.py

@@ -0,0 +1,30 @@
+"""SaaS stub extension tests."""
+
+from sandboxer.extensions.saas_stub import SaaSStubExtension
+from sandboxer.models import Profile
+
+
+def _profile() -> Profile:
+    return Profile.model_validate(
+        {"id": "profile.saas-stub", "version": "1.0.0", "extension": "ext.saas-stub"}
+    )
+
+
+def test_estimate_and_meter() -> None:
+    ext = SaaSStubExtension({"rate_usd_per_hour": 1.0, "session_fee_usd": 0.05})
+    quote = ext.estimate_cost(_profile(), {}, duration_s=3600)
+    assert quote is not None
+    assert quote.estimated_usd == 1.05
+
+    handle = ext.provision(_profile(), {}, "saas")
+    actual = ext.meter_actual(handle, duration_s=1800)
+    assert actual == 0.55
+
+
+def test_provision_wait_teardown() -> None:
+    ext = SaaSStubExtension()
+    handle = ext.provision(_profile(), {}, "saas-stub")
+    reach = ext.wait_ready(handle)
+    assert reach["endpoint"].startswith("https://stub.sandboxer.local/")
+    report = ext.teardown(handle)
+    assert report["provider_removed"] == "true"