store: replace bytes.NewBuffer+binary.Write with binary.BigEndian.PutUint64 in encodeSortableInt64

[Copilot]

encodeSortableInt64 was using bytes.NewBuffer + binary.Write to encode an int64 — introducing a buffer allocation and reflection overhead for a hot, low-level path.

Changes

• store/list_helpers.go: Replace bytes.NewBuffer + binary.Write with binary.BigEndian.PutUint64, writing directly into the caller-provided dst slice with zero allocations.

// Before
buf := bytes.NewBuffer(dst[:0])
_ = binary.Write(buf, binary.BigEndian, seq^math.MinInt64)

// After
binary.BigEndian.PutUint64(dst, uint64(seq^math.MinInt64))

---

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[github-actions]

🚫 [golangci] _{reported by reviewdog 🐶}
G115: integer overflow conversion int64 -> uint64 (gosec)

[bootjp]

@copilot

[store/list_helpers.go](https://github.com/bootjp/elastickv/pull/346/files/2d154bbf6fbfe55bc48c7b9aefde5d9fcbe72a8a#diff-715a1b973fd34b80452d5d8c2d31d72ddb3d181d329a8b15ae900ecb0d8f8f10)
	// binary.Write cannot fail here: the payload is a fixed-size int64 and the
	// destination is an in-memory bytes.Buffer.
	_ = binary.Write(buf, binary.BigEndian, seq^math.MinInt64)
	binary.BigEndian.PutUint64(dst, uint64(seq^math.MinInt64))
@[github-actions](https://github.com/apps/github-actions)
github-actions bot
[1 minute ago](https://github.com/bootjp/elastickv/pull/346#discussion_r2932213358)
🚫 [golangci] reported by [reviewdog](https://github.com/reviewdog/reviewdog) 🐶
G115: integer overflow conversion int64 -> uint64 (gosec)