Switch SMT node hashing to the v6a construction

[jait91]

Implements yellowpaper Appendix C.3.2: internal nodes now commit to their
absolute region — H(0x01 || depth || region_32B || left || right). Leaf
hashes and proof shapes unchanged; verifiers recompute the region from the
proven key, so nothing new is transmitted.

• Regions are derived, never persisted: from the inserted key at creation,
  from NodeKey + compressed path at load (guarded by
  key.DepthBits() + path.Len() == depth), preserved across restructures.
• Empty-tree root is now the all-zero hash (spec ⊥, matches the SDKs).
• TreeLayout bumped to yellowpaper-rsmt-sha256-v6a: pre-v6a data refuses
  to open — all existing trees (incl. perf snapshots) need a rebuild from
  finalized history.

Vectors (internal/smt/v6a_interop_vectors_test.go): empty, one-leaf,
shallow split, deep split (diverge at bit 255), multi-leaf, JS parity.
Verified against the JS SDK (unicitynetwork/state-transition-sdk-js#130) in
both directions — Go reproduces their root; their SparseMerkleTree
reproduces all five of ours. Java (unicitynetwork/state-transition-sdk-java#76)
and Rust (unicitynetwork/state-transition-sdk-rust#14) must reproduce at least
the deep-split and multi-leaf roots (multi-byte region packing) before the
coordinated rollover.

Closes #167

[gemini-code-assist]

Code Review

This pull request updates the Sparse Merkle Tree (SMT) implementation to comply with the yellowpaper v6a specification. Key changes include modifying the internal-node hash to incorporate the node's absolute key prefix (region), updating the empty tree root hash to be all-zero, and adding helper functions to derive these regions. Tests, golden vectors, and inclusion certificate verification have been updated accordingly, and a new interop test suite has been added to ensure parity with the JS SDK. A review comment identifies a bug in RegionFromKeyBytes where masking is incorrectly applied to a shallower byte if the input key is shorter than the expected depth, potentially clearing bits that should be preserved.

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