query: account for cursor-held buffers after Top-N compaction

[dantengsky]

Problem

TransformSortMergeLimit compacts retained DataBlocks after heap cursors have been built. With string sort keys, cursor rows can keep the pre-compaction StringView backing buffers alive while num_bytes reflects only the compacted block. This can understate the resident memory used by local Top-N.

Follow-up to #20186.

Follow-up

• Track cursor-held row memory per retained input block, or rebuild cursors from compacted sort rows.
• Remove the corresponding accounting when a block is evicted, and reset it when the heap is drained.
• Add regression coverage with synthetic sparse StringView sort keys.
• Preserve the no-copy fast paths for discarded, dense, and non-string blocks.
• Verify that spill decisions use the full retained-memory estimate.