Builds late-bound Delegation Context for Subagent child Turns.

Delegation Context is model-visible operational metadata for one child Turn. It stays out of Pixir's stable prompt prefix: callers pass the returned map to Pixir.Turn.run/3, which renders it into the late developer-context input item. The map is string-keyed so it can be tested, logged later if a future ADR accepts a durable event, and rendered deterministically without leaking Elixir struct details.