[skip changelog] Optimize dag stat with DP caching

[sneaxhuh]

Summary

Implements dynamic programming cache for CID traversal in ipfs dag stat to avoid redundant traversals when multiple DAGs share common subgraphs.

Resolves TODO in core/commands/dag/stat.go:17-18

Changes

Core Implementation

• Added cidStatCache structure to memoize subtree statistics for each CID
• Replaced linear traversal with recursive DP algorithm that:
  • Checks cache before traversing any node
  • Skips entire subtree traversal for cached CIDs
  • Computes and caches subtree stats (size + block count) for each node
• Maintains correct accounting for TotalSize, SharedSize, and deduplication ratios

Testing

• Added TestDagStatCaching with two test cases:
  • Cache consistency when querying duplicate CIDs
  • Correct deduplication stats with shared subgraphs

Performance Impact

Time Complexity:

• Before: O(N × M) where N = unique CIDs, M = times each appears
• After: O(N) - each unique CID traversed once

[gammazero]

Triage: Will review. This may be useful for pinner and for dag import, possibly by caching different data. Review should assess additional use cases.