Add Microsoft Store MSIX packaging workflow

[StephanOrgiazzi]

What this adds

• a separate Microsoft Store MSIX manual workflow;
• reproducible MSIX staging with MakeAppx.exe;
• a Partner Center-driven manifest template;
• exact Store icon assets generated during packaging;
• an optional packaged Everything Service (EverythingNext);
• a without_service fallback if Microsoft does not approve the restricted service capabilities;
• release documentation.

The existing NSIS/GitHub release workflow and Tauri updater are unchanged.

Required repository variables

Before running the workflow, configure the exact values from Partner Center:

• MSIX_IDENTITY_NAME
• MSIX_PUBLISHER
• MSIX_PUBLISHER_DISPLAY_NAME

The generated MSIX is intentionally unsigned because Microsoft signs Store MSIX packages after certification.

Validation note

The packaging script and manifest were validated statically here; a Windows runner is still required to execute cargo tauri build and makeappx.exe, then the resulting package should be tested in a clean Windows 11 VM before submission.

[chatgpt-codex-connector]

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