Each serialized result entry now carries cost_usd (rounded to 6 dp)
and cost_status (the child's session_cost_status — 'estimated',
'reported', 'included', or 'unknown') alongside tokens/api_calls/
duration, so the parent model can see what each delegation cost.
The internal _child_cost_usd field is still stripped before
serialization and the parent session cost rollup is untouched.
Tool schema is unchanged (byte-stable).
Inspired by: Perplexity Agent API result shape (idea-level)
Teknium