fix(sdk): harden BitNet package injection conditions in GlobalPackages.props

[Copilot]

This PR adds the new ANcpLua.NET.Sdk.BitNet variant and tightens the implicit BitNet hosting package injection so restore/build behavior stays safe across CPM and non-CPM projects. It also aligns asset flow to avoid silently dropping package-provided build assets.

• BitNet SDK variant

  • Adds ANcpLua.NET.Sdk.BitNet as a first-class SDK package (alongside base/Web/Test).
  • Wires generator + solution + packaging metadata for the new variant.

• Implicit BitNet hosting injection

  • Adds conditional auto-injection of ANcpLua.Agents.Hosting.BitNet for BitNet SDK consumers.
  • Applies the same self-reference protection pattern used elsewhere by excluding projects whose PackageId is ANcpLua.Agents.Hosting.BitNet.

• Asset inclusion safety

  • Updates the explicit asset list handling to avoid excluding buildMultitargeting assets in both CPM and non-CPM branches.

<ItemGroup Condition="
  '$(ANcpLuaSdkName)' == 'ANcpLua.NET.Sdk.BitNet'
  and '$(DisableANcpLuaBitNetHosting)' != 'true'
  and '$(PackageId)' != 'ANcpLua.Agents.Hosting.BitNet'">
  <PackageReference Include="ANcpLua.Agents.Hosting.BitNet" PrivateAssets="all" />
</ItemGroup>

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