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.

extensions/ext.saas-stub.yaml

@@ -0,0 +1,15 @@
+id: ext.saas-stub
+title: SaaS sandbox stub
+description: >
+  Metered SaaS extension stub for payments and routing v0. No external API;
+  use for burst fallback testing before E2B/Modal adapters land.
+handler: sandboxer.extensions.saas_stub:SaaSStubExtension
+capabilities:
+  isolation_levels: [microvm]
+  regions: [us, eu]
+  persistence: true
+  pricing_model: metered
+config:
+  provider: saas-stub
+  rate_usd_per_hour: 0.12
+  session_fee_usd: 0.01