New library (uc2_merkle.h / uc2_merkle.c) for Phase 3: - 64-bit FNV-1a content hashing for chunk addressing - Merkle tree: file -> list of chunk hashes -> root hash - Structural similarity comparison and shared chunk counting - Root hash changes on any content change (integrity) - Single-byte change affects only 1-2 chunks (locality) 8 unit tests including partial overlap and change resilience.