Modernize MIST for .NET 10 and replace PowerShell install scripts with NuGet build integration

[Copilot]

MIST was stuck on the legacy NuGet 2.x install model: a PowerShell Install.ps1 mutated the consumer's .csproj to register the IL-weaving build task, and target frameworks stopped at net8.0. This change extends framework coverage through .NET 10 and moves integration to the modern build/ + buildTransitive/ props/targets convention so a plain <PackageReference> is enough.

Target frameworks

• Mathtone.MIST (attribute library): netstandard2.0;net472;net6.0;net8.0;net9.0;net10.0.
• Mathtone.MIST.Builder (task assembly): multi-targeted net472 (full-framework MSBuild) + net8.0 (.NET SDK); selected at build time via $(MSBuildRuntimeType).

NuGet integration (replaces the PowerShell scripts)

• New build/Mathtone.MIST.Builder.props + .targets, auto-imported by NuGet, register <UsingTask> and run NotificationWeaverBuildTask AfterTargets="Build".
• Task assemblies pack under tasks/<tfm>/ via TargetsForTfmSpecificContentInPackage (per-TFM packing).
• Existing buildTransitive/ files forward to build/, so transitive references work too.
• Two opt-in/out MSBuild properties exposed:

<PropertyGroup>
  <MathtoneMistEnabled>true</MathtoneMistEnabled>           <!-- default true -->
  <MathtoneMistDebugMode>$(DebugSymbols)</MathtoneMistDebugMode>
</PropertyGroup>

Consumer install collapses to:

dotnet add package Mathtone.MIST
dotnet add package Mathtone.MIST.Builder

Removals

• Mathtone.MIST.Builder.Scripts/ (Install.ps1, Uninstall.ps1, .pssproj).
• Tests/Mathtone.MIST.Tests.NugetScriptTest/ — its sole purpose was validating the PS install path.
• Leftover Mathtone.MIST.Builder.csproj.old.
• Corresponding entries in Mathtone.MIST.sln.

Incidental fixes

• NotificationWeaver.FindPdbPathFor returned a bare filename with no directory, which broke under dotnet build (where the task's cwd ≠ output dir) and would have blocked the new auto-integration. Now combines the assembly's directory with the pdb name.
• .gitignore: whitelist Mathtone.MIST.Builder/build/ and buildTransitive/ (the catch-all build/ rule was hiding the new package files); ignore .nuget/ bootstrapped by net472 restore.
• README rewritten around the new flow; fixed SuppressNotify typo (was SupressNotify).