Add optimal AVX2/SSE reduction and refactor native SIMD code

* Replace icelake-server gcc target with skylake-avx512 in build script

* Remove global mutable state: eliminate initialIndexRegister,
  indexIncrement, maskSeventhBit, maskEighthBit globals and their
  constructor initializer; move mask constants (maskSeventhBit,
  maskEighthBit) to local scope inside lookup_partial_sums

* Add shared reduce_add_128_ps and reduce_add_256_ps helper functions
  using proper horizontal-add sequences instead of store-to-array loops

* Remove redundant if (length >= N) guards in all SIMD kernels — the
  loop body already handles the zero-iteration case correctly

* Replace store-to-aligned-array horizontal reduction pattern with the
  new helpers across all 128- and 256-bit dot product and euclidean
  distance functions

* Remove preferred_size parameter from dot_product_f32 and
  euclidean_f32; always dispatch to AVX-512 when length >= 16

* Standardize inline annotations: replace __attribute__((always_inline))
  inline with JV_FINLINE / JV_INLINE macros throughout

Author: Raghuveer Devulapalli

jvector-native/src/main/c/jextract_vector_simd.sh

@@ -49,7 +49,7 @@ CURRENT_GCC_VERSION=$(gcc -dumpversion)
 # Check if the current GCC version is greater than or equal to the minimum required version
 if [ "$(printf '%s\n' "$MIN_GCC_VERSION" "$CURRENT_GCC_VERSION" | sort -V | head -n1)" = "$MIN_GCC_VERSION" ]; then
     rm -rf ../resources/libjvector.so
-    gcc -fPIC -O3 -march=icelake-server -c jvector_simd.c -o jvector_simd.o
+    gcc -fPIC -O3 -march=skylake-avx512 -c jvector_simd.c -o jvector_simd.o
     gcc -fPIC -O3 -march=x86-64 -c jvector_simd_check.c -o jvector_simd_check.o
     gcc -shared -o ../resources/libjvector.so jvector_simd_check.o jvector_simd.o
 
@@ -77,4 +77,4 @@ jextract \
   jvector_simd.h
 
 # Set critical linker option with heap-based segments for all generated methods
-sed -i 's/DESC)/DESC, Linker.Option.critical(true))/g' ../java/io/github/jbellis/jvector/vector/cnative/NativeSimdOps.java
+sed -i 's/DESC)/DESC, Linker.Option.critical(true))/g' ../java/io/github/jbellis/jvector/vector/cnative/NativeSimdOps.java