[Question] StreamingMerge Entrypoint

Hi!

The FreshDiskANN paper outlines the StreamingMerge procedure. In combing through the codebase (main @ f8ef303), there doesn't appear to be a singular entrypoint that allows a caller to utilize the FreshDiskANN API contract without being aware of all the types of indices.

- `test_streaming_scenario.cpp` outlines how to build an in-memory index that supports inserts and deletes.
- `build_stitched_index.cpp` outlines how to merge indices
- `search_disk_index.cpp` demonstrates how to run a search across an index that is stored on disk.

Given a client that provides a memory budget and _no starting list of vectors_, my reading of the paper would indicate the following needs to be done in a wrapping class:

1. create an empty, streaming enabled, in-memory index that holds writes - this is outlined in `test_streaming_scenario.cpp` and is the only sink for insertions.
2. create an empty, SSD resident index, which is demonstrated by `build_disk_index.cpp`... this index would not have a true build phase as there is nothing to add.
3. once the mutable index in [1] is full, merge [1] and [2] using the routine outlined in `merge_shards` within disk_utils.h - during the merge process, we would have already created a new mutable in-memory index for any in-flight writes + deletes.
4. separately, maintain a list of deletions that are used for filtering within all live indices.

I would be happy to submit a patch that unifies the above in such a way that a caller can just create an Index and not have to worry about RO-TempIndex, RW-TempIndex and the SSD-Resident Index; however, I would like to confirm that my read on the current codebase is correct in that there is no singular entrypoint for this.

Start by reading test_streaming_scenario.cpp, build_stitched_index.cpp, build_disk_index.cpp, search_disk_index.cpp, and the merge_shards routine in disk_utils.h. Trace how the mutable, SSD-resident, and deletion-filtering pieces interact, then confirm whether the existing code lacks a unified entrypoint and define the wrapper's completion criteria with maintainers.

Written by the indexing model from the issue text.