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_routing.py

@@ -0,0 +1,59 @@
+"""Extension routing tests."""
+
+
+import pytest
+
+from sandboxer.extensions.registry import load_extension
+from sandboxer.models import Profile, RouteSpec, RouteStrategy
+from sandboxer.routing.resolver import resolve_extension
+
+
+def _burst_profile() -> Profile:
+    return Profile.model_validate(
+        {
+            "id": "profile.burst-sandbox",
+            "version": "1.0.0",
+            "extension": "ext.compose-ssh",
+            "route": {
+                "strategy": "prefer-self-hosted",
+                "extensions": ["ext.compose-ssh", "ext.saas-stub"],
+            },
+        }
+    )
+
+
+def test_explicit_uses_profile_extension() -> None:
+    profile = Profile.model_validate(
+        {
+            "id": "profile.compose-e2e",
+            "version": "1.0.0",
+            "extension": "ext.compose-ssh",
+        }
+    )
+    ext = resolve_extension(profile, {}, host_override="coulombcore")
+    assert ext.id == "ext.compose-ssh"
+
+
+def test_prefer_self_hosted_when_host_set(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("SANDBOXER_HOST", "coulombcore")
+    ext = resolve_extension(_burst_profile(), {"repo": "/tmp/x"}, host_override=None)
+    assert ext.id == "ext.compose-ssh"
+
+
+def test_prefer_self_hosted_falls_back_to_saas(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.delenv("SANDBOXER_HOST", raising=False)
+    monkeypatch.setenv("SANDBOXER_FORCE_SAAS", "1")
+    ext = resolve_extension(_burst_profile(), {}, host_override=None)
+    assert ext.id == "ext.saas-stub"
+    assert ext.capabilities.pricing_model == "metered"
+
+
+def test_lowest_cost_picks_metered_when_forced(monkeypatch: pytest.MonkeyPatch) -> None:
+    profile = _burst_profile()
+    profile.route = RouteSpec(
+        strategy=RouteStrategy.LOWEST_COST,
+        extensions=["ext.compose-ssh", "ext.saas-stub"],
+    )
+    monkeypatch.setenv("SANDBOXER_FORCE_SAAS", "1")
+    ext = resolve_extension(profile, {}, host_override=None)
+    assert load_extension(ext.id).capabilities.pricing_model == "metered"