fix(cascade): duplicate last merkle node on odd levels to match LEP-5

[kaleababayneh]

Merkle Tree Commitment Root Mismatch

buildTree in src/cascade/commitment.ts promotes an odd node to the next level unchanged.

The canonical implementation instead duplicates the last node so it pairs with itself: lumera/x/action/v1/merkle/merkle.go, which SuperNodes use for verification via pkg/cascadekit/commitment.go.

Because of this, any file whose chunk count produces an odd tree level commits a wrong availability_commitment root on-chain. The SuperNode recomputes the root from the received file, gets a different value, and rejects the upload:

LEP-5 commitment root verification failed: merkle root mismatch

Power-of-two chunk counts are not affected, which is why the existing tests passed. Actions already registered with a wrong root cannot complete and only refund at expiry.

The fix duplicates the last node when a tree level has an odd number of nodes, matching merkle.BuildTree.

Tests: the odd-leaves case now asserts the duplicate-pair root, plus gold vectors generated from the Go implementation (1–8 chunk trees and four buildCommitment cases including challenge indices). The odd-level cases fail against the old code and pass with the fix. full suite 93/93.

[Copilot]

Pull request overview

This PR fixes a Merkle tree root mismatch in the cascade LEP-5 availability commitment. Previously, buildTree promoted an odd node to the next level unchanged, whereas the canonical on-chain Go implementation (merkle.BuildTree) duplicates the last node so it pairs with itself. This caused files with chunk counts producing an odd tree level to commit a wrong availability_commitment root, leading SuperNodes to reject uploads with a "merkle root mismatch". Power-of-two chunk counts were unaffected, which is why prior tests passed.

Changes:

• Duplicate the last node (and store the padded level) when a tree level has an odd node count, matching the Go merkle.BuildTree.
• Update the odd-leaves test to assert the duplicate-pair root.
• Add parity tests with gold vectors generated from the production Go code (1–8 chunk trees and four buildCommitment cases including challenge indices).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/cascade/commitment.ts	Duplicates the last node on odd levels so hashing pairs it with itself, matching the canonical on-chain Merkle build.
tests/cascade/commitment.test.ts	Rewrites the odd-leaves assertion and adds Go-parity gold-vector tests for buildTree and buildCommitment.

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