fix(diskann): add diskann into zvec core lib (#602)

Author: richyreachy

src/core/CMakeLists.txt

@@ -60,14 +60,16 @@ if(NOT RABITQ_SUPPORTED)
   list(FILTER ALL_CORE_SRCS EXCLUDE REGEX ".*/algorithm/hnsw_rabitq/.*")
 endif()
 
-# Always exclude algorithm/diskann implementation files from zvec_core.
-# The DiskAnn algorithm is provided by the separate core_knn_diskann library
-# (STATIC+SHARED, real on Linux x86_64, stub on other platforms). The static
-# variant is whole-archived into _zvec.so for the Python wheel; the shared
-# variant is used by C++ tools and tests. Including the sources here would
-# cause duplicate symbols.
-list(FILTER ALL_CORE_SRCS EXCLUDE REGEX ".*/algorithm/diskann/.*")
+# Exclude algorithm/diskann implementation files from zvec_core when not
+# supported (matching the hnsw_rabitq pattern above). When DISKANN_SUPPORTED,
+# the diskann sources are packed into zvec_core directly, so libzvec_core.so
+# includes diskann symbols without needing a separate whole-archive step.
+# The standalone core_knn_diskann library (built by algorithm/diskann) is still
+# whole-archived into _zvec.so; since _zvec.so links zvec_core *normally*
+# (not --whole-archive), the linker skips zvec_core's diskann objects there
+# (symbols already resolved by core_knn_diskann_static) — no duplicates.
 if(NOT DISKANN_SUPPORTED)
+  list(FILTER ALL_CORE_SRCS EXCLUDE REGEX ".*/algorithm/diskann/.*")
   list(FILTER ALL_CORE_SRCS EXCLUDE REGEX ".*/interface/indexes/diskann_index\\.cc")
 endif()