feat: add hash commands

[kacy]

Summary

• adds Value::Hash(HashMap<String, Bytes>) variant to the core type system
• implements 10 hash commands: HSET, HGET, HGETALL, HDEL, HEXISTS, HLEN, HINCRBY, HKEYS, HVALS, HMGET
• includes memory tracking with proper overhead estimation for hash entries
• adds snapshot and recovery support for hash type persistence

Test plan

• keyspace unit tests for all hash operations
• command parsing tests for all 10 hash commands
• type safety tests (hash operations on non-hash keys return WRONGTYPE)
• auto-delete behavior when hash becomes empty via HDEL
• HINCRBY edge cases (overflow, non-integer values)
• all 450+ existing tests continue to pass
• clippy passes with no warnings

Design considerations

• hash memory overhead estimated as 64 bytes per entry + 48 bytes base, following the pattern established for other collection types
• empty hashes are auto-deleted (matching redis behavior) to prevent memory leaks
• HSET returns only the count of NEW fields (updates don't increment), matching redis semantics