Switch to AnyCPU for libraries, migrate to slnx

[devkanro]

Summary

Switch to per-project platform declarations: library and WPF projects default to AnyCPU,
WinUI application projects explicitly declare x64;ARM64. Migrate from .sln to .slnx.
Remove shared Platform fallback logic from Directory.Build.props.

Linked issue

Fixes #185

Key changes

Build system

• Directory.Build.props: Remove Platform fallback logic — only shared config (SDK version) remains
• Library projects (Reactor, Reactor.Interop.WinForms, Chat.UI, Chat.Model, PerfBench.Shared, StressPerf.Shared, CmdPerf.Shared): no <Platforms> tag, MSBuild defaults to AnyCPU
• WPF projects (StressPerf.Wpf, CmdPerf.Wpf): no <Platforms> tag — WPF has no native Windows App SDK dependency
• SelfTests: set WindowsAppSDKSelfContained=false — pure MSTest runner, no WinUI dependency
• App/test/sample projects: explicit <Platforms>x64;ARM64</Platforms> for WindowsAppSDKSelfContained native assets

Solution migration

• Reactor.sln → Reactor.slnx: cleaner XML format, solution platforms reduced to x64 and ARM64
• slnx platform mappings removed for AnyCPU-only projects

CI workflows

• ci.yml: pass explicit -p:Platform=x64 for standalone project builds
• coverage.yml: pass explicit -p:Platform=x64; replace hardcoded bin/x64 path with dynamic file lookup
• release.yml: remove -p:Platform=x64 from solution build/pack (slnx provides it); keep --runtime for publish

Code fixes

• Update FindHostExe() repo-root lookups in SelfTestBatch, TestSession, WinFormsTestSession, and CompileCommand to search for Reactor.slnx instead of Reactor.sln

Docs

• Update AGENTS.md, CONTRIBUTING.md, README.md, SKILL.md, PR template, devtools skill

Design

Each project declares its own platform requirements — no shared magic:

Project type	Platforms	Why
Library (Reactor, Interop, shared libs)	AnyCPU (default)	IL is always architecture-neutral; no native assets
WPF (StressPerf.Wpf, CmdPerf.Wpf)	AnyCPU (default)	No Windows App SDK dependency
Analyzer/Generator (netstandard2.0)	AnyCPU (inherent)	netstandard2.0
SelfTests (MSTest runner)	AnyCPU (default)	Pure test runner, launches AppTests.Host as subprocess
WinUI App/Test/Sample	x64;ARM64	WindowsAppSDKSelfContained needs concrete arch for native DLLs

Solution builds get Platform from the .slnx configuration. Standalone project builds
(e.g. dotnet test tests/Reactor.Tests) require explicit -p:Platform=x64.

Packaging verification

NuGet package — architecture-neutral, same structure as before:

Microsoft.UI.Reactor.nupkg
├── analyzers/dotnet/cs/
│   ├── Reactor.Analyzers.dll              (44.5KB)
│   └── Reactor.Localization.Generator.dll (16KB)
├── lib/net10.0-windows10.0.22621/
│   ├── Reactor.dll                        (2500KB, AnyCPU)
│   ├── Reactor.pri / Reactor.xml
│   └── Reactor/Hosting/ReactorApplication.xaml
└── LICENSE

CLI skill kit — RID-specific layout unchanged:

reactor-skill-kit-<version>.zip
└── reactor/
    ├── SKILL.md, skills/, install-skill-kit.ps1
    ├── bin/x64/    ← mur.exe (158.5KB) + 21 managed DLLs
    └── bin/arm64/  ← mur.exe (136.5KB) + 21 managed DLLs

Test plan

• dotnet build Reactor.slnx — 0 errors
• dotnet test tests/Reactor.Tests -p:Platform=x64 — 6819 passed, 46 skipped
• dotnet test tests/Reactor.SelfTests -p:Platform=x64 — 670 passed
• dotnet build Reactor.slnx -c Release — succeeds
• dotnet pack src/Reactor/Reactor.csproj — correct nupkg structure
• dotnet publish --runtime win-x64 / win-arm64 — both produce valid mur.exe
• bin/x64/mur.exe mirror layout still works

Risk / breaking changes

• Developers who used dotnet build/test <project> without -p:Platform for app projects will now need to either build via the solution or pass -p:Platform=x64 explicitly.
• The .sln file is removed; VS / Rider users need to open Reactor.slnx instead.

[devkanro]

@microsoft-github-policy-service agree company="microsoft"

[codemonkeychris]

Please check the nuget packaging - i think i ended up with x64/arm64 branches in there for the deployment of mur.exe (CLI devtool)... just check that the package generated correctly. @dotMorten has been dogfooding the nuget package from the builds, so I don't want to break him.

[devkanro]

Verified locally — packaging is unchanged.

NuGet package (framework):

Microsoft.UI.Reactor.nupkg
├── analyzers/dotnet/cs/
│   ├── Reactor.Analyzers.dll              (44.5KB)
│   └── Reactor.Localization.Generator.dll (16KB)
├── lib/net10.0-windows10.0.22621/
│   ├── Reactor.dll                        (2500KB, AnyCPU)
│   ├── Reactor.pri                        (1.6KB)
│   ├── Reactor.xml                        (924KB)
│   └── Reactor/Hosting/ReactorApplication.xaml
└── LICENSE

No x64/ARM64 branches — Reactor.dll is architecture-neutral (ProcessorArchitecture: None). Same structure as before this PR.

CLI skill kit (mur.exe): release.yml still publishes with --runtime win-x64 and --runtime win-arm64 separately. Both produce a valid mur.exe + all managed DLLs:

reactor-skill-kit-<version>.zip
└── reactor/
    ├── SKILL.md
    ├── skills/
    ├── install-skill-kit.ps1
    ├── bin/x64/          ← mur.exe (158.5KB, native apphost) + 21 DLLs
    └── bin/arm64/        ← mur.exe (136.5KB, native apphost) + 21 DLLs

mur.dll (196KB) and all other managed DLLs are identical between the two; only mur.exe (the native apphost) differs.

[codemonkeychris]

lgtm