D3L / audits encoding (erasure root, exported & imported segments)

Follow-up task from PR #859: https://github.com/FluffyLabs/typeberry/pull/859

Implement D3L / audits encoding including:
- Erasure root calculation
- Exported segments handling
- Imported segments handling

Requested by: @tomusdrw
Reference: https://github.com/FluffyLabs/typeberry/pull/859#discussion_r

Start by reading PR #859 and its linked discussion, since this issue is explicitly a follow-up to that work. Identify the relevant entry points and expected encoding behavior there before implementing the three listed areas: erasure root calculation, exported segments, and imported segments. Done means all three D3L/audits encoding paths are implemented and covered by the expectations established in PR #859.

Written by the indexing model from the issue text.