fix(unixfs): iterative dir finalization

[fforbeck]

Optimization of Directory Processing in UnixFS

Problem

The finalize method in UnixFSDirectoryBuilder is the most likely place to cause stack overflow errors when processing directories with large numbers of files (100K+). While we couldn't directly simulate the specific error condition, analysis of the code structure indicates that the recursive approach used for directory traversal would be prone to call stack limitations when dealing with large directory structures.

Potential Solution

Refactored the finalize method to use an iterative approach instead of recursion:

1. Implemented a discovery phase that collects all directories without processing them
2. Added a separate phase to collect and process files in batches
3. Introduced a depth-based sorting mechanism to process directories from deepest to shallowest
4. Added batch processing for large file collections to improve performance
5. Optimized memory usage by maintaining clear mappings between paths and their corresponding links

Resolves: storacha/w3cli#215