WIP prefetch - #23901
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Adds an async prefetch pipeline for hybrid scan SplitScan tasks: per-split prefetch coroutines run on the same event loop as scan_node and read_chunk, offloading pruning and byte-range computation to the existing thread pool rather than a dedicated one. Pinned host memory is reserved via reserve_or_fail by default (HybridScanPrefetchMemoryMode. FAIL_FAST), never waiting on memory something else may need, with reserve_memory (WAIT) available as an opt-in alternative. Byte ranges are coalesced before issuing reads, and a per-producer ordering mechanism (HybridScanPrefetchOrderingMode.ORDERED, default) keeps reservation attempts in consumption order so a split due for consumption soon can't lose its reservation to one that isn't. PrefetchedByteRanges covers both HybridScanPassMode variants (a single combined batch for SINGLE_PASS, separate filter/payload batches for TWO_PASS). Demotion of pinned memory under pressure is deliberately left as a TODO for both memory modes rather than built speculatively.
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