fix(slnx): load Samples.Diagnostics as a project, excluded from solution build

[ANcpLua]

What

The Diagnostics sample had been added to ErrorOrX.slnx as loose <File> items (including its .csproj as a plain file), so IDEs loaded the files without any project context — no EOE analyzer/generator squiggles appeared. This replaces them with a proper <Project> entry excluded from the solution build:

<Project Path="samples/ErrorOrX.Samples.Diagnostics/ErrorOrX.Samples.Diagnostics.csproj">
    <Build Solution="*|*" Project="false" />
</Project>

The project now loads in the IDE for live diagnostics, while dotnet build ErrorOrX.slnx still skips it (its build fails by design — seven EOE diagnostics reported as errors). Also updates the Diagnostics README, which claimed the project was excluded from the slnx entirely.

Verification

• dotnet build ErrorOrX.slnx → Build succeeded, 0 Warning(s), 0 Error(s)
• dotnet build samples/ErrorOrX.Samples.Diagnostics/ → fails as intended, firing EOE003, EOE004, EOE006, EOE007, EOE020, EOE021, EOE024

Status: complete-and-verified. No package content changed — no release needed.

🤖 Generated with Claude Code

[coderabbitai]

Important

Review skipped

Review was skipped due to path filters

⛔ Files ignored due to path filters (2)

• ErrorOrX.slnx is excluded by none and included by none
• samples/ErrorOrX.Samples.Diagnostics/README.md is excluded by none and included by none

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⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 1c9f2ee9-117e-484a-81cf-aaa53662d372

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