Switch SMT inclusion verification from version 3o to 6a

[ristik]

Changed:

• src/radix.rs

  • Added prefix_region(key, depth) using the repo’s existing LSB-first radix convention.
  • Region = key bits < depth, split bit and later bits zeroed.

• src/api/inclusion_certificate.rs

  • Inclusion proof wire format unchanged.
  • Leaf hash unchanged.
  • Internal node hash now folds:
    • 0x01 || depth || region || left_hash || right_hash
  • Region is derived during verification from the proven key and sibling depth.
  • Added tests that v6a roots verify and old 3o roots do not.

• tests/transition_flow.rs

  • Existing JS-generated multi-leaf fixture is now treated as pre-v6a negative data until the JS fixture generator is updated.
  • Alice single-leaf fixture still verifies because single-leaf roots are unchanged.

Validation:

• cargo test --lib passed: 92/92.
• cargo test passed: full default suite.

[gemini-code-assist]

Code Review

This pull request updates the sparse-Merkle-tree inclusion path verification to incorporate canonical radix regions (prefix_region) when hashing internal nodes, aligning with the RSMT v6a specification. It also updates integration tests to reject pre-v6a multi-leaf fixtures while verifying single-leaf fixtures. Feedback was provided on src/radix.rs to add an assertion in prefix_region ensuring depth does not exceed 256, preventing potential out-of-bounds panics.

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[gemini-code-assist]

The prefix_region function takes depth: usize and performs indexing on a 32-byte array ([u8; 32]). If depth is greater than 256, full_bytes will be greater than or equal to 32, which will cause a panic due to out-of-bounds slicing (region[..full_bytes]) or out-of-bounds indexing (region[full_bytes]).

While the current caller in verify bounds depth to MAX_DEPTH (255), as a general pub(crate) helper function, it is safer and more robust to add an assertion to prevent potential panic-based DoS or unexpected out-of-bounds panics if it is ever called with larger depths in the future.

Suggested change

	pub(crate) fn prefix_region(key: &[u8; 32], depth: usize) -> [u8; 32] {
	let mut region = [0u8; 32];
	let full_bytes = depth / 8;
	let partial_bits = depth % 8;
	region[..full_bytes].copy_from_slice(&key[..full_bytes]);
	if partial_bits != 0 {
	let mask = (1u8 << partial_bits) - 1;
	region[full_bytes] = key[full_bytes] & mask;
	}
	region
	}
	pub(crate) fn prefix_region(key: &[u8; 32], depth: usize) -> [u8; 32] {
	assert!(depth <= 256, "depth cannot exceed 256");
	let mut region = [0u8; 32];
	let full_bytes = depth / 8;
	let partial_bits = depth % 8;
	region[..full_bytes].copy_from_slice(&key[..full_bytes]);
	if partial_bits != 0 {
	let mask = (1u8 << partial_bits) - 1;
	region[full_bytes] = key[full_bytes] & mask;
	}
	region
	}