Remove dead LAPACK code, fix Marshal.SizeOf pragma, convert hot-path dictionaries to FrozenDictionary

Dead code removal:
- Delete Backends/LAPACK/ directory (LAPACK.cs, LAPACK.NetLib.cs,
  np.LAPACK.cs) containing 6 DllImport P/Invoke declarations and the
  LAPACKProviderType enum — all dead code, called only from linear
  algebra stubs that return null/default (inv, qr, svd, lstsq)
- Delete test/NumSharp.UnitTest/LAPACK/ (dgels.cs, dgesvd.cs) — tests
  for the removed dead LAPACK infrastructure

Marshal.SizeOf fix:
- np.save.cs: Remove unnecessary #pragma warning disable 618 around
  Marshal.SizeOf(Type) — only the Marshal.SizeOf(Object) overload is
  obsolete, not the Type overload used here

FrozenDictionary conversion (System.Collections.Frozen, .NET 8+):
- np.find_common_type.cs: Convert all 4 type promotion dictionaries
  (_typemap_arr_arr, _nptypemap_arr_arr, _typemap_arr_scalar,
  _nptypemap_arr_scalar) from Dictionary to FrozenDictionary. These
  contain 180+ entries each and are hit on every arithmetic/comparison
  operation for type resolution. FrozenDictionary optimizes the hash
  table at freeze time for faster lookups.
- np.dtype.cs: Convert DType._kind_list_map from Dictionary to
  FrozenDictionary (14 entries, used in find_common_type).
- TypelessConvert.cs: Convert _delegates from Dictionary to
  FrozenDictionary (156 entries, boxing type-conversion lookup table).

Test results unchanged: 1601 passed, 50 failed on both net8.0/net10.0.
2 previously-skipped LAPACK tests removed with the dead code.

Author: Nucs

src/NumSharp.Core/APIs/np.save.cs

@@ -219,9 +219,7 @@ private static string GetDtypeFromType(Array array, out Type type, out int bytes
             }
             else
             {
-#pragma warning disable 618 // SizeOf would be Obsolete
-                bytes = Marshal.SizeOf(type);
-#pragma warning restore 618 // SizeOf would be Obsolete
+                bytes = System.Runtime.InteropServices.Marshal.SizeOf(type);
             }
 
             if (type == typeof(bool))

src/NumSharp.Core/Backends/LAPACK/LAPACK.NetLib.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Linq;
 using System.Numerics;
@@ -10,7 +11,7 @@ namespace NumSharp
     /// <remarks>https://docs.scipy.org/doc/numpy-1.16.0/reference/generated/numpy.dtype.html#numpy.dtype</remarks>
     public class DType
     {
-        protected internal static readonly Dictionary<NPTypeCode, char> _kind_list_map = new Dictionary<NPTypeCode, char>()
+        protected internal static readonly FrozenDictionary<NPTypeCode, char> _kind_list_map = new Dictionary<NPTypeCode, char>()
         {
             {NPTypeCode.Complex, 'c'},
             {NPTypeCode.Boolean, '?'},
@@ -26,7 +27,7 @@ public class DType
             {NPTypeCode.Single, 'f'},
             {NPTypeCode.Decimal, 'f'},
             {NPTypeCode.String, 'S'},
-        };
+        }.ToFrozenDictionary();
 
         /// <summary>Initializes a new instance of the <see cref="T:System.Object"></see> class.</summary>
         public DType(Type type)