[iaojnh]

Flat can now search externally owned row-major FP32 L2/IP vectors without storing a second copy of their payloads. VectorSource supplies an optional batched iterator for exhaustive scans; candidate searches reuse its existing random-access methods. Callers register IDs with add_with_source, then bind a source per search request.

The scan domain is the intersection of externally enumerated IDs, registered IDs, and the request filter. Callers must keep the source, scan domain, and visibility filter on the same snapshot. Scan batches can retain backing pages through a lease; fetched result vectors borrow source memory.

This includes request isolation, storage-mode validation, reopen coverage, and guards for unsupported formats and operations. Registration skips redundant FP32 payload conversion. Filtered or unregistered rows may omit vector pointers, and reusing query parameters preserves the visibility filter.

Validation: 210 related local tests passed, with 2 platform/size-dependent skips; all changed C++ files pass clang-tidy with warnings treated as errors. The Windows ARM64 C1001 failure is addressed by the upstream test fix from alibaba#795. This branch has also been merged with current main, preserving its memory-lifetime and read-only-close fixes and guarding the new internal-buffer fast path in external-vector mode.

The initial scope excludes quantized/sparse/column-major vectors, cosine, group-by, source-bound refinement, and dump/merge of external Flat indexes.

Related: alibaba#797.