Switch SMT inclusion verification from version 3o to 6a

Update the SMT code from the current 3o-style node commitment to the v6a construction from rsmt6a.py.

Scope:

• No backwards compatibility required; this is a coordinated pre-production rollover.
• Leaf hash stays unchanged: it still commits to the full key and value.
• Inclusion proof shape stays unchanged: bitmap/depth set + sibling hashes.
• Internal node hash changes to also commit to the node's canonical region/prefix.
• This repo does not need to generate or verify append-only consistency proofs.

Implementation notes:

• Old 3o node commitment was effectively:
  • H_node(depth, left_hash, right_hash)
• New v6a node commitment is:
  • H_node(depth, region, left_hash, right_hash)
• region is the canonical prefix for that node at depth, using this repo's existing key bit order, endian-ness, and packing conventions.
• During inclusion proof verification, no extra proof field is needed. Recompute region = prefix(proven_key, depth) at each sibling-combine step.

Acceptance:

• Update root/inclusion verification tests to v6a hashes.
• Verify shared interop vectors generated by aggregator-go, JS, Rust, and Java.
• Generate Java vectors for the other repos to verify.
• Include cases for empty tree, one leaf, shallow split, deep split, and multi-leaf tree.