refactor: extract SHA3 hashing utilities to utils/hash.go

- Move all SHA3-512 hashing functions to utils package
- Update import statements and function calls
- Maintain zero functional changes
- First step in systematic main.go refactoring

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

refactor.md

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+# Refactoring Proposal for KVS Main.go
+
+After analyzing your 3,990-line main.go file, I've identified clear functional areas that can be separated into manageable modules.
+Here's my comprehensive refactoring proposal:
+
+Proposed File Structure
+
+kvs/
+├── main.go                    # Entry point + minimal server setup
+├── config/
+│   └── config.go             # Configuration structures and loading
+├── types/
+│   └── types.go              # All data structures and type definitions
+├── auth/
+│   ├── auth.go               # Authentication & authorization logic
+│   ├── jwt.go                # JWT token management
+│   ├── middleware.go         # Auth middleware
+│   └── permissions.go        # Permission checking utilities
+├── storage/
+│   ├── storage.go            # BadgerDB operations and utilities
+│   ├── compression.go        # ZSTD compression/decompression
+│   ├── ttl.go                # TTL and metadata management
+│   └── revision.go           # Revision history system
+├── cluster/
+│   ├── gossip.go             # Gossip protocol implementation
+│   ├── members.go            # Member management
+│   ├── sync.go               # Data synchronization
+│   └── merkle.go             # Merkle tree operations
+├── server/
+│   ├── server.go             # Server struct and core methods
+│   ├── handlers.go           # HTTP request handlers
+│   ├── routes.go             # Route setup
+│   └── lifecycle.go          # Server startup/shutdown logic
+├── features/
+│   ├── ratelimit.go          # Rate limiting middleware and utilities
+│   ├── tamperlog.go          # Tamper-evident logging
+│   └── backup.go             # Backup system
+└── utils/
+    └── hash.go               # Hashing utilities (SHA3, etc.)
+
+Key Benefits
+
+1. Clear Separation of Concerns: Each package handles a specific responsibility
+2. Better Testability: Smaller, focused functions are easier to unit test
+3. Improved Maintainability: Changes to one feature don't affect others
+4. Go Best Practices: Follows standard Go project layout conventions
+5. Reduced Coupling: Clear interfaces between components
+
+Functional Areas Identified
+
+1. Configuration (~100 lines): Config structs, defaults, loading
+2. Types (~400 lines): All data structures and constants
+3. Authentication (~800 lines): User/Group/Token management, JWT, middleware
+4. Storage (~600 lines): BadgerDB operations, compression, TTL, revisions
+5. Clustering (~1,200 lines): Gossip, members, sync, Merkle trees
+6. Server (~600 lines): Server struct, handlers, routes, lifecycle
+7. Features (~200 lines): Rate limiting, tamper logging, backup
+8. Utilities (~90 lines): Hashing and other utilities
+
+Migration Strategy
+
+1. Start with the most independent modules (types, config, utils)
+2. Move storage and authentication components
+3. Extract clustering logic
+4. Refactor server components last
+5. Create commits for each major module migration
+
+The refactoring will maintain zero functional changes - purely cosmetic restructuring for better code organization.