fix(python): only bundle the DiskANN plugin where it is supported

The wheel install rule for the DiskANN plugin was gated on
`if(TARGET core_knn_diskann)`. That target always exists: on platforms where
DiskAnn is unsupported (macOS, ARM64, ...) it is built from an empty stub
(src/core/algorithm/CMakeLists.txt) with zero exported symbols, and the
runtime load path is compiled out via `#if DISKANN_SUPPORTED`. So the
condition was always true and a useless ~16 KB stub `.dylib` was shipped in
every non-Linux-x86_64 wheel.

Gate the install on `DISKANN_SUPPORTED` instead, so the plugin is packaged
only where it is real (currently Linux x86_64 with libaio).

Verified on macOS arm64: `libcore_knn_diskann.dylib` no longer appears in the
wheel; `import zvec` and other index types are unaffected.

Author: Cuiyus

CMakeLists.txt

@@ -179,9 +179,14 @@ if(BUILD_PYTHON_BINDINGS)
     # function. The Python extension resolves the module next to _zvec.so
     # (see the $ORIGIN rpath in src/binding/python/CMakeLists.txt); the
     # module must therefore be installed alongside _zvec.so in the wheel.
-    # The target exists only on platforms where DiskAnn is buildable
-    # (currently Linux x86_64 with libaio).
-    if(TARGET core_knn_diskann)
+    #
+    # Gate on DISKANN_SUPPORTED, not on the target's existence: on unsupported
+    # platforms (e.g. macOS / ARM64) the core_knn_diskann target is still
+    # defined, but built from an empty stub (src/core/algorithm/CMakeLists.txt)
+    # with zero exported symbols and a runtime load path compiled out
+    # (#if DISKANN_SUPPORTED). Shipping that stub is pure dead weight, so it is
+    # only packaged where DiskAnn is real — currently Linux x86_64 with libaio.
+    if(DISKANN_SUPPORTED)
         install(TARGETS core_knn_diskann LIBRARY DESTINATION ${ZVEC_PY_INSTALL_DIR})
     endif()
     # Bundle cppjieba's dictionary files so the `jieba` FTS tokenizer works