add comprehensive test suite for UnixFS CID determinism per IPIP-499:
- verify exact HAMT threshold boundary for both estimation modes:
- v0-2015 (links): sum(name_len + cid_len) == 262144
- v1-2025 (block): serialized block size == 262144
- verify HAMT triggers at threshold + 1 byte for both profiles
- add all deterministic CIDs for cross-implementation testing
also wires SizeEstimationMode through CLI/API, allowing
Import.UnixFSHAMTSizeEstimation config to take effect.
bumps boxo to ipfs/boxo@6707376 which aligns HAMT threshold with
JS implementation (uses > instead of >=), fixing CID determinism
at the exact 256 KiB boundary.
