Fix cross-namespace Nexus result delivery when the service result uses external storage.
The caller now receives verified encoded bytes through its own namespace's storage policy. It does not gain access to the service namespace's storage references. Same-namespace and inline results preserve their behavior. A failed caller storage write rolls back the original claim and terminal event so that the same task can be retried after storage recovers.
This correction applies to newly recorded terminal deliveries. It does not rewrite terminal history already persisted by 2.6.0; upgrading alone does not repair those histories.
See #846. Published Server and SDK acceptance is tracked in #753.