docs: Move iteration-roadmap to internal directory

Move iteration-roadmap.md to internal as it's primarily a design document for Phase 1 implementation (now complete) and future Phase 2/3 planning.

Rationale:
- iteration-layers.md provides sufficient public documentation
- roadmap is valuable as internal design record
- keeps public docs directory focused on user-facing content

Public documentation:
- docs/iteration-layers.md (architecture guide) ✅
- docs/evolution-guide.md (decision guide) ✅

Internal documentation:
- docs/internal/iteration-roadmap.md (design/planning) ✅

Author: NeilJo-GY