Implement BLAKE3 Merkle tree (#core-001)

[jvanulde]

Streaming Merkle over payload files; commit root in manifest.

[edip-dev-01]

Addressed in fork commit edip-dev-01/q-cap@33659d9: adds deterministic BLAKE3 payload Merkle tree computation and commits the resulting root into the manifest/signature flow. Pending upstream merge: edip-dev-01@33659d9

[edip-dev-01]

Closed by merged PR #39: #39