fix: ship desktop bundles without a Python DLL dependency

[RebelliousSmile]

What changed

• make the PyO3 bridge opt-in instead of part of the default desktop feature set
• add a regression test for self-contained default builds
• bump the corrective release metadata to 0.3.1
• remove the now-unnecessary PyO3 ABI workaround from the release workflow

Why

The v0.3.0 Windows executable imported python311.dll and failed to start on systems without Python 3.11. Production bundles must not depend on a system Python installation.

Validation

• 360 frontend tests passed
• 167 backend tests passed (2 network tests ignored)
• 3 Tauri tests passed, including the new packaging regression
• Tauri 0.3.1 Windows build succeeded without Python/PyO3 in the default dependency graph
• executable smoke test stayed running for five seconds without a missing-DLL error

Fixes #74