[CI logging] Add fixtures and GitHub Actions dogfood workflow

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### Summary

Add deterministic diagnostic fixtures and a GitHub Actions workflow for end-to-end CI logger validation.

### Background and Motivation

Unit tests can validate emitted workflow-command text, but the MVP must also prove that GitHub Actions recognizes the commands, displays annotations, and links them to the correct source locations.

The MSBuild repository is sufficient for this validation; a separate repository is not required.

### Proposed Feature

Add fixtures covering:

- compiler errors and warnings;
- an MSBuild-generated error;
- missing source locations;
- special characters and multiline messages;
- warning promotion;
- multiproject and parallel failures.

Add a locked-down GitHub Actions dogfood workflow that:

- starts as a manually triggered workflow;
- builds MSBuild first;
- runs the fixtures with bootstrap MSBuild;
- asserts expected success or failure and emitted workflow commands;
- runs on GitHub-hosted Linux and Windows;
- uses read-only permissions and no secrets.

Initial acceptance includes manual confirmation that annotations appear once and navigate to the expected repository file and location.

### Alternative Designs

A separate repository would add maintenance and version-coordination costs without improving MVP validation. Tests alone would not verify GitHub's native annotation rendering.

Aucun fichier ni test précis n’est indiqué ; commencez par localiser le workflow CI existant, les tests de journalisation diagnostique et le point d’entrée bootstrap de MSBuild dans le dépôt MSBuild. Ajoutez des fixtures déterministes ainsi qu’un workflow Linux et Windows en lecture seule, déclenché manuellement, qui valide les résultats attendus et les commandes du workflow, puis vérifiez que les annotations apparaissent une seule fois et renvoient vers l’emplacement source attendu.

Rédigé par le modèle d'indexation à partir du texte de l'issue.