Implement Atomic Pointer Activation + Success Lifecycle

[BLMgithub]

Implement atomic publish activation and success state transition.

Scope

• Update metadata.json → status = SUCCESS, completed_at set, published = true
• Write temporary pointer file (_latest.tmp)
• Payload:
  • run_id
  • version = v{run_id}
  • published_at (UTC)
• Atomically swap using os.replace → _latest.json

Execution order requirement

1. Integrity gate passes
2. Version copy completes
3. metadata.json updated to SUCCESS
4. Atomic pointer swap

Acceptance Criteria

• Pointer always references fully copied version
• No partial pointer writes possible
• Pointer never updated if metadata update fails
• Dashboard reads only _latest.json