fix(core): make Merkle root hashing deterministic

[wuyua9]

Summary

• build the Merkle DAG root hash from sorted file paths instead of Map insertion order
• add a regression test that builds identical file-hash maps in different insertion orders and checks the root id stays stable

Why

buildMerkleDAG() already adds child nodes in sortedPaths order, but the root node data was still built from the unsorted Map key order. When the same file hashes are inserted in a different traversal order, the root id can change even though the files did not.

MerkleDAG.compare() treats changed node ids as added/removed, so an insertion-order-only root change can make the synchronizer do unnecessary file-state comparisons and snapshot writes.

Tests

• cd packages/core && node_modules/.bin/jest src/sync/synchronizer.test.ts --runInBand
• ./node_modules/.bin/tsc --build packages/core --force
• git diff --check