Adds the trust-zone Origin to CaptureContext and the I1 gate to the
use-case: a confidential effective classification through a us-nexus
origin is refused before ANY write (ErrSovereigntyRefused), and the
refusal is itself audited. A caller-asserted harness label that names a
different zone than the server-derived origin is logged as a security
event — context.Harness is descriptive-only, never a gate input.
The gate triggers only on an explicit ZoneUSNexus, so the unset default
(ZoneUnknown) can never make it fire on caller-controllable input; the
REST adapter always sets a concrete zone.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>