[Enhance]: Follow up on DiskANN integration and validation

[zhourrr]

Affected Component

DiskANN

Current Behavior

Background

PR #602 packages the DiskANN implementation into zvec_core. This allows downstream consumers to use DiskANN without linking core_knn_diskann separately.

The change was merged promptly to unblock the upcoming Node.js SDK release. It has been validated downstream on Linux x86_64.

The following review comments should be addressed in a follow-up change.

Follow-up tasks

• Decide whether interface/indexes/diskann_index.cc should remain in zvec_core when DISKANN_SUPPORTED=OFF.

  • Consider aligning its behavior with HNSW RaBitQ.
  • If the interface can compile without the DiskANN implementation, keep the interface layer available and guard only implementation-specific code.
  • Verify behavior on supported and unsupported platforms.
  • Review context: discussion

• Simplify the CMake comments introduced by fix(diskann): add diskann into zvec core lib #602.

  • Keep a concise explanation of why DiskANN implementation sources are conditionally included.
  • Avoid documenting excessive linker implementation details in the build file.
  • Review context: discussion

• Add a core-level DiskANN example or integration test.

  • Link only against zvec_core.
  • Do not explicitly link core_knn_diskann.
  • Create a DiskANN index.
  • Insert documents.
  • Build or optimize the index.
  • Execute a vector query and validate its results.
  • Run the example or test on Linux x86_64 with DISKANN_SUPPORTED=ON.
  • Review context: discussion

• Confirm that Python binding linkage remains free of duplicate symbols.

  • The Python binding still whole-archives core_knn_diskann_static while also linking zvec_core.
  • Verify that supported Python wheel builds and imports continue to work correctly.

Desired Improvement

See above

Impact

See above