winui-dev-workflow + analyzer: set <RuntimeIdentifiers> for multi-arch packaging by default

[nmetulev]

97: #97

Problem

From Joe's blog post:

When I selected to package for x64 and Arm64 I got an error stemming from the runtime identifiers not being set, a quick fix.

The winui-mvvm template scaffolds a project without a <RuntimeIdentifiers> property. winapp package for both x64 and Arm64 then fails with an error that's clear once you know it, opaque if you don't. Joe figured it out manually; the agent didn't anticipate it.

Proposed fix

Two-part:

1. Scaffold fix (winui-dev-workflow) — after dotnet new winui-mvvm, ensure the .csproj contains:

   <PropertyGroup>
     <RuntimeIdentifiers>win-x64;win-arm64</RuntimeIdentifiers>
   </PropertyGroup>

   via a small post-scaffold merge step in the skill (or, ideally, a PR upstream to the WinUI template itself so this disappears).

2. Analyzer rule (Microsoft.WindowsAppSDK.Analyzers) — new rule (proposed ID WAPK00XX, Warning severity, with helpLinkUri):

   Package.appxmanifest declares Arm64 (or any architecture not present in <RuntimeIdentifiers>) — winapp package will fail. Add win-arm64 to <RuntimeIdentifiers> in the .csproj.

   This catches the existing-project case the scaffold fix doesn't cover.

Related

May be subsumed by — or coordinated with — a unified "greenfield polish phase" step (see cross-cutting themes on the 97 issue).

Acceptance criteria

• New scaffold passes winapp package for both x64 and arm64 with zero edits.
• Analyzer rule fires on a mismatched existing project; analyzer test added.
• Rule documented in src/tools/winui-analyzer/README.md with its rule-table entry.

Acknowledgments

Identified by Joe Finney in Building Fluent Icon Finder with the WinUI Copilot skill.