docs: fix DisaggregatedSet nits from #811 review (#911)

- Rename 'phase' → 'role' throughout concepts doc for terminology
  consistency (hasB4K)
- Correct rollout strategy ownership: DisaggregatedSet controller takes
  over rollout entirely; partition-based rollout is unsupported (hasB4K)
- Fix 'role isolation' claim: rollouts are coordinated across roles to
  preserve capacity ratios, not fully independent (hasB4K)
- Remove 'API Version' from Key Design Principles (hasB4K)
- Change 'controller' → 'operator' in overview/_index.md (hasB4K)

Follow-up to #811.