Document breaking change: Composite ML-DSA on Windows uses native implementation in .NET 11

[Copilot]

Starting in .NET 11, CompositeMLDsa on Windows switches from a managed implementation layered over ML-DSA/RSA/ECDSA to the native Windows (CNG) implementation, which currently supports only a subset of composite algorithms.

New breaking change article

• Added docs/core/compatibility/cryptography/11/compositemldsa-windows-native.md covering previous/new behavior, reason for change, and recommended action (checking CompositeMLDsa.IsAlgorithmSupported and handling PlatformNotSupportedException)
• Verified the exact set of natively-supported parameter sets against the BCRYPT_PQDSA_KEY_BLOB docs: 4 ECDSA-paired algorithms (MLDSA44-ECDSA-P256-SHA256, MLDSA65-ECDSA-P256-SHA512, MLDSA65-ECDSA-P384-SHA512, MLDSA87-ECDSA-P384-SHA512) — RSA-paired algorithms and EdDSA-paired algorithms are no longer natively supported on Windows

Index/TOC updates

• Added the article to docs/core/compatibility/toc.yml and the Cryptography table in docs/core/compatibility/11.md

Related doc update

• Updated the Composite ML-DSA support matrix in docs/standard/security/cross-platform-cryptography.md so Windows only shows support for the 4 natively-implemented algorithms, with a link to the new breaking change article for context

if (CompositeMLDsa.IsAlgorithmSupported(CompositeMLDsaAlgorithm.MLDsa65WithECDsaP384))
{
    using CompositeMLDsa mldsa = CompositeMLDsa.GenerateKey(CompositeMLDsaAlgorithm.MLDsa65WithECDsaP384);
    // Use mldsa.
}
else
{
    // Fall back to another algorithm, or handle the lack of support.
}

• Fixes [Breaking change]: Composite ML-DSA on Windows uses native implementation #54565

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Internal previews

File	Preview link
docs/core/compatibility/11.md	docs/core/compatibility/11
docs/core/compatibility/cryptography/11/compositemldsa-windows-native.md	docs/core/compatibility/cryptography/11/compositemldsa-windows-native
docs/core/compatibility/toc.yml	docs/core/compatibility/toc
docs/standard/security/cross-platform-cryptography.md	docs/standard/security/cross-platform-cryptography

[Copilot]

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[gewarren]

@copilot Please also update https://github.com/dotnet/docs/blob/main/docs/core/compatibility/unsupported-apis.md?plain=1.

[Copilot]

@copilot Please also update https://github.com/dotnet/docs/blob/main/docs/core/compatibility/unsupported-apis.md?plain=1.

Added CompositeMLDsa entries to unsupported-apis.md in e944835.

[BillWagner]

This LGTM, once you have the one question answered.