Fix shard distribution bias: restore default expiration, add WithMaxKeyLength docs, and getShardID tests

[Copilot]

Shard ID computation had a logic bug when maxKeyLength=0 (hashed empty string instead of full key), the 30s default expiration was silently dropped from New[V](), and the new WithMaxKeyLength option lacked documentation.

Changes

• Restore default expiration: New[V]() now applies WithDefaultExpiration[V](30 * time.Second) alongside WithMaxKeyLength[V](256) — its removal was an unintended breaking change
• Fix getShardID logic: normalize kl=0 to use the full key length before branching; clamp kl to min(len(key), kl) consistently across both paths
• Document WithMaxKeyLength: clarifies the 256-byte default, the maphash/xxh3 selection threshold (≤32 bytes vs longer), and the meaning of kl=0
• Add getShardID unit tests: covers all behavioral boundaries — kl=0, kl=1, kl∈[2,32], kl>32, key shorter than kl, prefix isolation, and result range invariants

// Before: kl=0 caused key[:0] (empty string) to be hashed → all short keys → same shard
// After: kl=0 falls through to full-key path correctly
func getShardID(key string, kl uint64) (id uint64) {
    if kl != 0 {
        kl = min(uint64(len(key)), kl)
        // ... maphash or xxh3 on key[:kl]
    }
    // full key path when kl == 0
}

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