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.

registry/capabilities/execution.sandbox-provision.md

@@ -16,27 +16,27 @@ maturity:
       Charter (INTENT.md), meta-framework spec, extension SDK, integration docs,
       and research synthesis. Capability indexed in registry/.
   availability:
-    current: A4
+    current: A5
     target: A5
     confidence: high
     rationale: >
       CLI v0 (create/destroy/snapshot/TTL), HTTP API, CoulombCore remote smoke.
-      SaaS stub + routing + credits shipped (SAND-WP-0006).
+      Cloud adapters E2B/Modal + routing + credits (SAND-WP-0010).
 
 external_evidence:
   completeness:
-    level: C4
-    name: Substantial
+    level: C5
+    name: Mature
     confidence: high
     basis: scope_vs_intent_and_consumer_expectations
     satisfied_expectations:
       - profile-based create/destroy/snapshot/restore via CLI
       - TTL extend and expire/reap (SAND-WP-0009)
+      - E2B and Modal cloud adapters with BYOK (SAND-WP-0010)
       - State Hub lifecycle events on transitions
       - wise-validator and the-custodian migration arc complete
-      - extension SDK with compose-ssh, vm-packer attach, saas-stub
+      - extension SDK with compose-ssh, vm-packer attach, saas-stub, e2b, modal
     broken_expectations:
-      - Real E2B/Modal adapters not yet built
       - sandboxer01 dedicated host not live (CoulombCore interim)
     out_of_scope_expectations:
       - agent harness and tool orchestration (glas-harness)