Publish inject-trigger and occurrence contracts

[Brad-Edwards]

Expose the accepted general inject-trigger operation independently of participant-directed projection delivery.

Acceptance criteria

• Publish the trigger request, occurrence, target/content binding, correlation, authorization context, duplicate handling, and execution/readback contracts.
• Keep general trigger and occurrence identity independent of participant/control identity. Provide correlation for consumers that implement participant-directed bindings.
• Carry shared operation outcomes and backend refusal without converting acceptance into delivery or observation evidence.
• Update affected public APIs, schemas, Python contracts, processor bindings where required, and migration/publication records. Include a participant-free environment and external-trigger example.

Prerequisites: #1353, #1360.

References: Environment and inject diagnosis.