kv: fix ReverseScanAt cross-shard ordering when clampToRoutes=false

[Copilot]

reverseScanRoutesAt with clampToRoutes=false (used for S3 manifest scans spanning multiple shards) concatenated per-route results with a shrinking limit, producing out-of-order output and silently dropping keys when shard ranges interleave in descending keyspace.

Changes

• reverseScanRoutesAt: when clampToRoutes=false, fetch limit results from every route then merge+sort descending — mirroring the approach scanRoutesAt already uses for the ascending case. When clampToRoutes=true, preserve the original short-circuit on len(out) >= limit.
• mergeAndTrimReverseScanResults: new helper symmetric to mergeAndTrimScanResults but sorting descending (bytes.Compare > 0).

// before: concatenates per-route results with shrinking budget → wrong order
for i := len(routes) - 1; i >= 0 && len(out) < limit; i-- {
    kvs, _ := s.scanRouteAtDirection(ctx, route, ..., limit-len(out), ts, true)
    out = append(out, kvs...)
}

// after: fetch limit from every route, merge+sort descending
kvs, _ := s.scanRouteAtDirection(ctx, route, ..., limit, ts, true)
out = mergeAndTrimReverseScanResults(out, kvs, limit)

Tests

New tests added in kv/shard_store_test.go:

• TestShardStoreReverseScanAt_DescendingOrderAcrossShards: verifies that a nil-start scan (clampToRoutes=false) across two shards returns all keys merged in descending order.
• TestShardStoreReverseScanAt_LimitAcrossShards: verifies that the top-N keys are correctly selected across shard boundaries (e.g. limit=4 from 3+3 keys picks the 4 largest).
• TestShardStoreReverseScanAt_SingleShard: single-shard baseline for descending order.
• TestShardStoreReverseScanAt_IncludesS3ManifestKeysDescending: mirrors the existing forward-scan manifest test — reverse scan over manifest keys returns descending order.
• TestMergeAndTrimReverseScanResults / _EmptyInput / _WithinLimit / _ExactLimit: unit tests for the new helper covering merge, descending sort, and limit trimming edge cases.

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[bootjp]

@copilot add test code

[Copilot]

@copilot add test code

Added in commit d6fd20f. Eight tests covering:

• TestShardStoreReverseScanAt_DescendingOrderAcrossShards — nil start (clampToRoutes=false) merges both shards in descending order
• TestShardStoreReverseScanAt_LimitAcrossShards — top-N selection across shards (e.g. limit=4 from 3+3 keys picks top 4 descending)
• TestShardStoreReverseScanAt_SingleShard — single-shard baseline
• TestShardStoreReverseScanAt_IncludesS3ManifestKeysDescending — manifest key reverse scan returns descending order
• TestMergeAndTrimReverseScanResults / _EmptyInput / _WithinLimit / _ExactLimit — unit tests for the new helper