Default-initialize SVSStorageKind in IndexSVSVamana (#5054)

Summary:
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/5054

`IndexSVSVamana::storage_kind` was declared without a default initializer,
and the default constructor is `= default`, so the field was left
uninitialized in default-constructed instances. This is undefined behavior
any time the value is read — including serialization via `write_index`,
which writes the garbage value to disk.

Add `= SVS_FP32` as the default initializer, matching the default used by
the parameterized constructor `IndexSVSVamana(d, degree, metric, storage)`.

This is a safe, behavior-preserving change:

- The parameterized constructor already defaults to `SVS_FP32`, so any
  code constructing an index with arguments is unaffected.

- The two derived classes (`IndexSVSVamanaLVQ`, `IndexSVSVamanaLeanVec`)
  explicitly set `storage_kind` in their own default constructors, so
  they are also unaffected.

- The only code path that changes behavior is default construction of
  `IndexSVSVamana` itself, which previously produced an uninitialized
  (UB) value and now produces `SVS_FP32`.

Reviewed By: mnorris11

Differential Revision: D99891611

fbshipit-source-id: da6acff7bdeb5668a2bf5f3b585bc1a3179004b9

faiss/svs/IndexSVSVamana.h

@@ -86,7 +86,7 @@ struct IndexSVSVamana : Index {
     size_t max_candidate_pool_size = 200;
     bool use_full_search_history = true;
 
-    SVSStorageKind storage_kind;
+    SVSStorageKind storage_kind = SVS_FP32;
 
     IndexSVSVamana();