feat|test: Add eigSH for symmetric/Hermitian eigendecomposition

Adds `eigSH` via a new `af_eigsh` C wrapper (cbits/eigsh.c) that calls
cuSOLVER on CUDA backends and falls back to SVD on CPU/OpenCL.  Includes
unit and property-based tests covering eigenvalue ordering, eigenvector
orthonormality, and full matrix reconstruction.  Also fixes minor test
description duplicates in ArithSpec and ArraySpec.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dmjio

arrayfire.cabal

@@ -87,6 +87,7 @@ library
     af
   c-sources:
     cbits/wrapper.c
+    cbits/eigsh.c
   build-depends:
     base < 5, deepseq, filepath, vector
   hs-source-dirs: