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.

src/sandboxer/core/manager.py

@@ -21,6 +21,7 @@ from sandboxer.models import (
     SandboxStatus,
     SnapshotRecord,
 )
+from sandboxer.payments.billing_export import export_meter_usage
 from sandboxer.payments.credits import CreditsStore
 from sandboxer.payments.metering import estimate_cost, settle_usage
 from sandboxer.placement import resolve_host
@@ -60,6 +61,8 @@ class SandboxManager:
             "vm_target": status.inputs.get("vm_target", ""),
             "vm_host": status.inputs.get("vm_host", ""),
             "endpoint": status.inputs.get("endpoint", ""),
+            "provider_sandbox_id": status.inputs.get("provider_sandbox_id", ""),
+            "provider": status.inputs.get("provider", ""),
         }
 
     def _resolved_host(self, profile, extension, host_override: str | None) -> str:
@@ -133,6 +136,8 @@ class SandboxManager:
             status.inputs["vm_target"] = handle.get("vm_target", "")
             status.inputs["vm_host"] = handle.get("vm_host", "")
             status.inputs["endpoint"] = handle.get("endpoint", "")
+            status.inputs["provider_sandbox_id"] = handle.get("provider_sandbox_id", "")
+            status.inputs["provider"] = handle.get("provider", "")
             reach = backend.wait_ready(handle)
             status.reachability = Reachability(**reach)
             status.state = SandboxState.READY
@@ -209,6 +214,7 @@ class SandboxManager:
         if settled and settled.pricing_model == "metered" and settled.actual_usd:
             self.credits.debit(settled.actual_usd)
             status.meter = settled
+            export_meter_usage(status, extension_id=extension.id, meter=settled)
             emit_lifecycle_event(
                 status,
                 summary=(