Add mermaid diagrams to Xet protocol specification docs
- index.md: block diagram showing overall Xet architecture (file → chunks → xorbs → shard → CAS)
- xorb.md: packet diagram for chunk header wire layout (replaces ASCII art)
- shard.md: packet diagrams for all binary structures — header, FileDataSequenceHeader/Entry, FileVerificationEntry, FileMetadataExt, CASChunkSequenceHeader/Entry, footer (replaces ASCII art)
- chunking.md: flowchart for the CDC boundary decision algorithm
- hashing.md: flowchart showing the four hash computation paths (chunk, xorb, file, verification)
- file-id.md: sequence diagram for the resolve URL → X-Xet-Hash flow
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
