Trial Nx affected/cache + per-project OS routing for monorepo builds

[Redth]

As maui-labs grows into a true monorepo (DevFlow, CLI, Linux GTK4, samples, integration tests), the per-product workflow split is starting to feel coarse: a touch in Directory.Packages.props rebuilds every product, and shared infra like eng/** or global.json invalidates everything regardless of what actually depends on it. This PR sets up a non-gating trial of an Nx + @nx/dotnet driven affected/cache pipeline alongside the existing CI so we can measure whether it pays off before committing to a migration.

Nothing here changes the existing ci-devflow, ci-cli, ci-linux-gtk4, or devflow-integration workflows — they remain the source of truth for green PRs. Everything new is opt-in telemetry.

What's in here

Nx workspace plumbing

• Adds Nx 22.7.1 + @nx/dotnet 22.7.0 with checked-in nx / nx.bat wrappers and a small Node bootstrap (eng/nx/nx-msbuild-resolvers.js) that exports the MSBuild SDK resolver environment so @nx/dotnet's evaluator can find Microsoft.DotNet.Arcade.Sdk from the dnceng feeds.
• Restricts nx.json targetDefaults to --no-restore (drops the inferred --no-dependencies), with restore added to build.dependsOn. The fallback was needed because no-op multi-TFM ProjectReferences fail GetTargetPath resolution under --no-dependencies.

Eliminated project.json files (7 of them). Most metadata that was hand-rolled into project.json is now inferred by @nx/dotnet from MSBuild itself, or expressed as real ProjectReference edges, or moved out of the graph entirely.

• Skill checks (plugins/dotnet-maui, tests/dotnet-maui, .github/plugin) were already invoked directly by their own workflows; removed from the graph.
• samples/DevFlow.Sample/ and Microsoft.Maui.Cli.UnitTests/ are now fully inferred.
• Integration tests previously needed a project.json for per-platform test:android|ios|maccatalyst|windows targets. Replaced with four per-platform csprojs (Android/, iOS/, MacCatalyst/, Windows/) each importing a shared IntegrationTests.Common.props. Sources stay shared via <Compile Include="..\*.cs" />. Platform identity is stamped into each assembly via [AssemblyMetadata("DevFlowTestPlatform", ...)] so AppFixtureFactory no longer needs an env var. The Sample is built via a real (no-op) ProjectReference from each platform test project, which means the dependency edge appears in the Nx graph instead of needing implicitDependencies.

NxBuildableOn for self-declared OS routing. Each project owns where it can run via an MSBuild property:

<PropertyGroup>
  <NxBuildableOn>linux</NxBuildableOn>            <!-- Linux.Gtk4 family -->
  <NxBuildableOn>macos</NxBuildableOn>            <!-- iOS / MacCatalyst integration tests -->
  <NxBuildableOn>windows</NxBuildableOn>          <!-- Windows integration tests -->
  <!-- Default when unset: linux;macos;windows  -->
</PropertyGroup>

The new eng/nx/nx-os-tags.js (createNodesV2) reads it from the csproj first, then walks Directory.Build.props files up to the workspace root, and stamps os:<name> tags onto each project. Defaults to all three OSes plus an os:any umbrella tag, matching the "managed-only code can build anywhere" reality (an -ios class library still gets os:any even though it targets Apple APIs).

Two new workflows, both continue-on-error: true:

• nx-affected-report.yml: lists the affected projects and ships a Linux cache canary using raegen/nx.
• nx-build-shadow.yml: matrix on Linux, macOS, and Windows. Each runner does affected -t restore/build/test --projects=tag:os:${{ matrix.os-tag }}. No centralized exclusion list; the only special case is skipping platform integration tests because those need real devices/sims handled by devflow-integration.yml.

Verified locally

• ./nx show projects returns 23 projects, all inferred (zero project.json files in the tree).
• ./nx show projects --projects=tag:os:macos returns 19 (Apple ITs + every os:any).
• ./nx run Microsoft.Maui.DevFlow.Agent.IntegrationTests.MacCatalyst:test builds the Sample for net10.0-maccatalyst via the no-op ProjectReference, launches the app, and runs 113 tests (111 pass; 2 unrelated UI-property flakes).
• A Driver local cache canary correctly restored artifacts/bin/obj from cache after deletion.

Things to keep an eye on

• raegen/nx remote cache behavior on actual GitHub runs is still untested; the shadow workflow exists specifically to surface that signal.
• The tags plugin reads <NxBuildableOn> via regex, so values inside conditional <PropertyGroup Condition="..."> blocks are picked up regardless of the condition. If you need a value that depends on $(TargetFramework) etc., put it directly in the csproj where you have that context.
• nx.json targetDefaults.build now omits --no-dependencies globally. Nx's ^build chain still orders things; MSBuild's incremental check makes the redundant resolution cheap, but worth watching on cold-cache CI runs.
• Integration tests against Android, iOS, and Windows lanes have not been re-validated locally after the per-platform csproj split — only MacCatalyst was exercised end-to-end. The shadow workflow will be the first signal here.

Marking draft because this is a trial — happy to keep iterating based on what the first few CI runs surface.