Implement RefineExternalitiesImpl

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

Machine-related work to be split into multiple PRs.

Currently, `RefineExternalitiesImpl` is a stub implementation with all methods throwing "Method not implemented." errors. This task involves implementing the actual machine-level operations including:
- [ ] Machine expunge, pages management, void/zero pages
- [ ] Memory peek/poke operations
- [ ] Machine initialization and invocation
- [ ] Segment export
- [ ] Historical lookups

Each point should be handled separately as a sub-task.

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

Start with RefineExternalitiesImpl and PR #859, then identify how its stub methods map to the listed machine operations. Split the work into separate subtasks for pages and expunge, memory access, initialization and invocation, segment export, and historical lookups; done means each selected operation no longer throws a not-implemented error and has appropriate coverage.

Written by the indexing model from the issue text.