Follow-up hardening for the FastCDC chunking feature (#67). Current gaps: no explicit backwards-compatibility matrix (both transfer directions), no measured performance metrics, only ad-hoc adversarial coverage, and no fuzzing (cross-impl agreement rests on a single golden vector).

Adds:

• Backwards-compat matrix: chunked↔basic in BOTH directions, no regression to basic (REQ-LFS-015).
• Chunker/manifest robustness via property + fuzz tests (REQ-LFS-016).
• Fuzz-backed client/server FastCDC agreement across many randomized inputs (REQ-LFS-017).
• Performance metrics with rigorous methodology: chunking throughput, bytes-over-the-wire by change type, storage dedup ratio (REQ-LFS-018).