[Spec 027] Prove canonical Runtime B to Router to Runtime A lineage

[XnLemon]

Part of Spec 027 and linked to parent Issue #59. Extend the backend cross-runtime acceptance with the canonical reverse direction: Runtime B receives the root managed request, invokes Runtime A through the Agent SDK and A2A Router, and returns through the original Gateway request. Assert one root Task and Trace, parent/child Invocation linkage, exact immutable Release provenance for both Agents, direct endpoint isolation, and distinct failure semantics. No new runtime framework, direct database mutation, retry, alternate endpoint, or contract fallback.

[XnLemon]

Slice C scope clarification: the acceptance now includes the existing Runtime B sample's explicit Agent SDK caller configuration and a deterministic Runtime A responder capability runtime.echo while preserving Runtime A's runtime.cross caller path. Runtime B still has no direct Runtime A endpoint; both hops use Router Agent v1, separate Router Agent credentials, and no Agent host ports. The E2E asserts exactly one B root plus one A child, shared rootTaskId/traceId, parentInvocationId, distinct published Release/Card digest provenance, immutable Release reads, and direct endpoint rejection. No new public contract, runtime framework, retry, fallback, or alternate route is in scope.

[XnLemon]

Resolved in PR #62 commit 2d1b4c1. Root cause was the Console browser acceptance fixture wiring the two internal trust-boundary digests in the wrong direction: Control Plane -> Router now matches NEKIRO_ROUTER_INTERNAL_BEARER_TOKEN, and Router -> Control Plane now matches NEKIRO_CONTROL_PLANE_SERVICE_TOKEN. No runtime, contract, retry, fallback, or alternate endpoint changes. CI run 30319275997 passed all 7 checks, including production console-browser-acceptance.

[XnLemon]

Reverse lineage verification: root PR #62 commit 2d1b4c1 corrected the browser fixture internal-auth digest direction. CI run 30319275997 passes all 7 checks, including backend-acceptance and console-browser-acceptance. The reverse B -> Router -> A acceptance and independent review are recorded in Spec 027 tasks; review result was 0 High and 0 Medium. No runtime contract, retry, fallback, alternate endpoint, or direct Agent path was added. Process note: Slice C and Slice D are separate commit scopes but share PR #62, recorded as open T039.