[Settings] Eliminate XamlIndexBuilder parallel build race

[Yu Leng (moooyo)]

Summary of the Pull Request

Eliminates the Settings XamlIndexBuilder parallel-build race by making the tool a build-only ProjectReference owned by PowerToys.Settings.

The change also:

• removes the nested MSBuild invocation and cross-platform obj directory deletion
• isolates tool and generated-index outputs by platform and configuration
• generates and registers the index before AssignTargetPaths so clean builds embed it on the first pass
• models inputs and outputs for MSBuild incrementality and Visual Studio's fast up-to-date check
• prevents build-tool assets from flowing into the Settings product output

PR Checklist

• Tests: targeted Settings solution-filter builds pass
• Localization: no end-user-facing strings changed
• Dev docs: no documentation change required
• New binaries: no new binaries added

Detailed Description of the Pull Request / Additional comments

PowerToys.Settings and Settings.UI.XamlIndexBuilder were both scheduled independently by the solution, while Settings also invoked a second AnyCPU build of the same project. The nested build's AfterBuild target deleted obj/x64 while the solution build was still using it, causing MSB3231 and CS2012 failures.

The builder is now a pure build tool. A non-runtime ProjectReference establishes one dependency in the MSBuild graph and exposes the tool output to a Settings-owned generation target. Generated JSON is kept under a platform/configuration-specific obj directory and copied to the same Assets/Settings destination as before.

Validation Steps Performed

• Built src/settings-ui/PowerToys.Settings.slnf in Debug x64 with parallel MSBuild and restore using the configured host NuGet source.
• Repeated the Debug x64 build to verify the generation target is skipped when its inputs and output are up to date.
• Confirmed build.debug.x64.errors.log is empty and the logs contain no MSB3231, CS2012, or NETSDK1150 errors.
• Confirmed the generated JSON parses with 467 entries and matches the copied output byte-for-byte.
• Confirmed PowerToys.Settings.dll contains Microsoft.PowerToys.Settings.UI.Assets.search.index.json.
• Confirmed no XamlIndexBuilder DLL, deps.json, runtimeconfig.json, or PDB is copied into the Settings product output.