NumSharp.Bitmap: add [SupportedOSPlatform], comprehensive tests, and OpenBugs

Source changes (np_.extensions.cs, NumSharp.Bitmap.csproj):
- Add [SupportedOSPlatform("windows")] attribute to np_ class, marking the
  library as Windows-only since System.Drawing.Common wraps GDI+ which is
  only available on Windows since .NET 6. This enables CA1416 analyzer
  warnings for cross-platform consumers.
- Update package description to clarify Windows-only + GDI+ dependency.

New test coverage (BitmapExtensionsTests.cs — 52 tests):
- ToNDArray: dtype, contiguity, size consistency, discardAlpha (copy/no-copy),
  flat output, pixel data correctness, odd width handling, copy vs no-copy
  data equality, multiple embedded resources (captcha-a through captcha-d,
  odd-width).
- ToBitmap: 32bpp round-trip, auto format detection (24bpp/32bpp), flat
  input reshaping, error handling (null, wrong ndim, format mismatch,
  multiple pictures), size property verification, specific pixel value
  round-trip fidelity.
- Image.ToNDArray: delegation to Bitmap overload.
- AsNDArray: BitmapData zero-copy wrapper (shaped, with discardAlpha).
- ToBytesPerPixel: all supported pixel formats, unsupported formats,
  DontCare rejection.
- Synthetic images: zeros/ones images at various sizes.

OpenBugs.Bitmap.cs (8 bugs, 11 failing tests):
  Bug 1 (GH #396): ToNDArray(copy:true) computes bytes-per-pixel as
    stride/width — wrong when GDI+ stride includes 4-byte alignment
    padding. 3px wide 24bpp: stride=12, stride/width=4 instead of 3.
    5px wide: stride=16, 32 bytes copied but reshape expects 30 →
    IncorrectShapeException.
  Bug 2 (GH #440): Same root cause on 2px wide 24bpp: stride=8,
    stride/width=4 instead of 3, producing (1,H,W,4) shape.
  Bug 3: AsNDArray(flat:true) accesses shape[3] on the 1-d flat array
    before reshaping to 4-d → IndexOutOfRangeException. Both with and
    without discardAlpha.
  Bug 4 (GH #475): ToBitmap on sliced (non-contiguous) NDArray fails
    in MultiIterator.Assign — flat vector can't broadcast against the
    sliced 4-d shape → IncorrectShapeException.
  Bug 5 (GH #491): ToBitmap with non-byte dtype (e.g. int32) throws
    cryptic InvalidCastException from CopyTo<byte>. No dtype guard or
    auto-conversion.
  Bug 6 (GH #440): ToBitmap stride padding logic concatenates a zeros
    column when stride != width*bpp. This is fundamentally wrong — GDI+
    pads rows to 4-byte boundaries by bytes, not by adding pixel columns.
    The concat inflates nd.size beyond the destination buffer →
    ArgumentOutOfRangeException. Tested at 1px and 5px wide.
  Bug 7: 24bpp 2px-wide round-trip produces corrupted pixel data because
    Bug 2's wrong bpp shifts channel boundaries. SetPixel(1,0)=(40,50,60)
    recovers as (0,40,50).
  Bug 8: ToBitmap with unsupported channel count (e.g. 2) lets
    extractFormatNumber() fall through with format=DontCare, producing
    an unhelpful GDI+ "Parameter is not valid" error instead of a
    descriptive ArgumentException.

Two architectural root causes explain most bugs:
  1. Copy path uses stride/width for bpp instead of deriving from
     PixelFormat — the no-copy path correctly uses ReshapeFlatData().
  2. ToBitmap tries to handle stride padding by concatenating columns
     instead of copying row-by-row with per-row padding.

Author: Nucs

src/NumSharp.Bitmap/NumSharp.Bitmap.csproj

@@ -5,7 +5,7 @@
     <PackageRequireLicenseAcceptance>true</PackageRequireLicenseAcceptance>
     <Authors>Eli Belash, Haiping Chen, Meinrad Recheis</Authors>
     <PackageOutputPath>../../packages</PackageOutputPath>
-    <Description>This package provides extensions for System.Drawing.Bitmap for creating NDArray and Bitmap with or without copying.</Description>
+    <Description>Windows-only extensions for System.Drawing.Bitmap for creating NDArray and Bitmap with or without copying. Uses GDI+ via System.Drawing.Common.</Description>
     <PackageProjectUrl>https://github.com/SciSharp</PackageProjectUrl>
     <Copyright>2021 © SciSharp STACK Team</Copyright>
     <RepositoryUrl>https://github.com/SciSharp/NumSharp</RepositoryUrl>

src/NumSharp.Bitmap/np_.extensions.cs

@@ -2,12 +2,14 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Drawing;
 using System.Drawing.Imaging;
+using System.Runtime.Versioning;
 using NumSharp.Backends;
 using NumSharp.Backends.Unmanaged;
 
 // ReSharper disable once CheckNamespace
 namespace NumSharp
 {
+    [SupportedOSPlatform("windows")]
     [SuppressMessage("ReSharper", "SuggestVarOrType_SimpleTypes")]
     [SuppressMessage("ReSharper", "UnusedMember.Global")]
     public static class np_