[geaflow/ai-memory] Add minimal GeaFlow backend vertical slice design doc

Priority: P1
Difficulty: Starter

Context: GeaFlow is a streaming graph engine, but geaflow-ai currently cannot treat the main engine as an implemented Graph Memory backend. A vertical slice must be scoped before coding.

Scope:

- Write a design document for a minimal `GeaFlowBackend` slice.
- Define supported operations, non-goals, checkpoint behavior, and test strategy.
- Map to existing GeaFlow DSL/runtime components.

Constraints:

- No production HA claim.
- No implementation in this issue.
- Must identify exact missing APIs or blockers.

Acceptance Criteria:

- Maintainers can decide whether the first slice is read-only, write-through, or projector-based.
- Design includes rollback plan and compatibility constraints.

Start by mapping the existing GeaFlow DSL and runtime components relevant to a Graph Memory backend. The design document should define supported operations, non-goals, checkpoint behavior, test strategy, exact missing APIs or blockers, compatibility constraints, and a rollback plan so maintainers can choose the first slice.

Written by the indexing model from the issue text.