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.

src/sandboxer/extensions/base.py

@@ -6,7 +6,7 @@ import uuid
 from abc import ABC, abstractmethod
 from typing import Any
 
-from sandboxer.models import Profile
+from sandboxer.models import MeterQuote, Profile
 
 
 class SandboxExtension(ABC):
@@ -31,4 +31,18 @@ class SandboxExtension(ABC):
 
     @abstractmethod
     def teardown(self, handle: dict[str, str]) -> dict[str, str]:
-        """Release sandbox resources. Returns cleanup report fields."""
+        """Release sandbox resources. Returns cleanup report fields."""
+
+    def estimate_cost(
+        self,
+        profile: Profile,
+        inputs: dict[str, str],
+        *,
+        duration_s: int = 3600,
+    ) -> MeterQuote | None:
+        """Optional pre-create cost quote (metered SaaS extensions)."""
+        return None
+
+    def meter_actual(self, handle: dict[str, str], *, duration_s: float) -> float | None:
+        """Optional post-destroy actual cost in USD."""
+        return None