fix: correct Pebble iterator bounds and use pre-allocated txn prefix bytes

[Copilot]

Two issues in the previous PR: iterator bounds in GetAt/LatestCommitTS leaked adjacent user keys into the scan range, and txnLockKey/txnIntentKey weren't using the pre-allocated prefix byte slices.

Iterator upper-bound bug (store/lsm_store.go)

With the [userKey][8-byte inverted TS] encoding, the old upper bound encodeKey(key, 0) + 0x00 = key + \xFF×8 + \x00 does not confine iteration to a single user key. Any key where key is a byte-prefix leaks in — e.g. for key="a", all versions of "ab" satisfy "ab\xFF…" < "a\xFF…\x00" and fall inside the range.

Adds a keyUpperBound helper (next lexicographic prefix, standard LSM idiom) and applies it to both GetAt and LatestCommitTS:

// before — too wide; "ab" versions bleed into "a"'s range
upper := encodeKey(key, 0)
upper = append(upper, 0)

// after — tightly confines to exactly userKey
UpperBound: keyUpperBound(key),  // e.g. "a" → "b", "a\xFF" → "b", all-\xFF → nil

In practice the userKey equality check after SeekGE/First would catch the mismatch, but incorrect bounds mean Pebble may scan unnecessary SST blocks, and conflict checks in LatestCommitTS could return a false negative if the seek lands on a prefix-adjacent key first.

txn prefix byte slices (kv/txn_keys.go)

txnLockKey and txnIntentKey were appending the string constants directly instead of the pre-allocated txnLockPrefixBytes/txnIntentPrefixBytes slices introduced in the same prior commit:

// before
k = append(k, txnLockPrefix...)   // string constant

// after
k = append(k, txnLockPrefixBytes...)  // pre-allocated []byte

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[bootjp]

@copilot このPRの変更にかかわらず、 golangci-lint をパスさせてください

[Copilot]

@copilot このPRの変更にかかわらず、 golangci-lint をパスさせてください

Fixed all 23 golangci-lint issues in commit e27d5ab (plus e6ea2b8 for the gci auto-fixes):

• gci (3): Auto-fixed import formatting in kv/lock_resolver.go, kv/sharded_coordinator_abort_test.go, store/lsm_store.go
• unused (1): Removed unused withinBoundsKey from store/mvcc_store.go
• forcetypeassert (1): Comma-ok assertion in store/lsm_store_test.go
• wrapcheck (2): Wrapped ctx.Err() with errors.WithStack() in kv/lock_resolver.go and store/lsm_store.go
• mnd (4): Extracted uint64FieldSize constant in kv/txn_codec.go and txnNamespaceSentinel in kv/lock_resolver.go
• gosec G115 (2): Changed goroutine loop variables to uint64 in store/mvcc_store_concurrency_test.go
• cyclop (9): Extracted helper functions across cmd/server/demo.go, kv/fsm.go, store/lsm_store.go, store/mvcc_store.go, and test files
• gocognit (1): Extracted writer/scanner helpers in TestMVCCConcurrentScanAtAndPutAt

Local golangci-lint v2.11.4 now reports 0 issues.