feat: cloud adapters E2B/Modal and billing export (SAND-WP-0010)

Add credentialed E2B and Modal extensions, burst routing fallback,
fin-hub meter export hook, BYOK docs, and 77 tests.

tests/test_routing.py

@@ -16,7 +16,12 @@ def _burst_profile() -> Profile:
             "extension": "ext.compose-ssh",
             "route": {
                 "strategy": "prefer-self-hosted",
-                "extensions": ["ext.compose-ssh", "ext.saas-stub"],
+                "extensions": [
+                    "ext.compose-ssh",
+                    "ext.e2b",
+                    "ext.modal",
+                    "ext.saas-stub",
+                ],
             },
         }
     )
@@ -42,12 +47,23 @@ def test_prefer_self_hosted_when_host_set(monkeypatch: pytest.MonkeyPatch) -> No
 
 def test_prefer_self_hosted_falls_back_to_saas(monkeypatch: pytest.MonkeyPatch) -> None:
     monkeypatch.delenv("SANDBOXER_HOST", raising=False)
+    monkeypatch.delenv("E2B_API_KEY", raising=False)
+    monkeypatch.delenv("MODAL_TOKEN_ID", 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_prefer_self_hosted_falls_back_to_e2b_when_credentialed(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setenv("SANDBOXER_FORCE_SAAS", "1")
+    monkeypatch.setenv("E2B_API_KEY", "test-key")
+    ext = resolve_extension(_burst_profile(), {}, host_override=None)
+    assert ext.id == "ext.e2b"
+
+
 def test_lowest_cost_picks_metered_when_forced(monkeypatch: pytest.MonkeyPatch) -> None:
     profile = _burst_profile()
     profile.route = RouteSpec(