Execute externally triggered injects through the shared runtime

[Brad-Edwards]

An accepted external trigger must create and execute a real inject occurrence with an independently evidenced world-effect outcome.

Acceptance criteria

• Implement the public trigger path, authorized admission, durable correlation, backend dispatch, refusal, and truthful outcome readback under the accepted inject contracts.
• Execute the authored inject content against the selected world target. Queuing a name or publishing participant status alone cannot count as realization.
• Persist world-effect outcomes independently of participant disclosure, delivery, and observation. Expose occurrence correlation for participant-specific consumers.
• Verify a participant-free world inject, duplicate submission, refusal, crash/reconciliation, and failed execution through an executing backend adapter. Keep fixtures distinct from concrete backend evidence.

Prerequisites: #1366, #1359, #1362.

References: Diagnosis, Environment and inject diagnosis.