Parent: #3858

Background

Current SnapshotBuilder traversal in crates/rooch-pruner/src/state_prune/snapshot_builder.rs uses BFS with VecDeque (pop_front/push_back). For large and/or wide SMT graphs, the queue can grow very large (O(width) / potentially O(n)), which increases peak memory and may reintroduce memory pressure even after streaming writes.

Goal

Change traversal to DFS using a stack (Vec) to bound the worklist size closer to O(depth) (SMT depth is typically small and bounded), reducing peak memory usage.

Scope

• Replace VecDeque BFS queue with Vec stack in traversal.
• Keep snapshot semantics unchanged (still visits all reachable nodes).
• Keep current dedup strategy and writer behavior unchanged (this is a focused memory fix).

Acceptance

• Traversal uses DFS stack (Vec) and does not allocate a large VecDeque queue.
• No correctness regression: snapshot completeness preserved.
• Existing tests pass.