ci: support private Windows test drafts

[McanKul]

Summary

• add an opt-in stage_test_build mode to the existing Windows release workflow
• derive a unique draft tag from the exact source SHA
• keep test builds as owner-only draft releases with explicit non-release and no-redistribution notes
• preserve the existing version-release path unchanged when the option is disabled

Why

Issue #14 needs packaged Windows retesting with private real-device HEIC files. Public Actions artifacts are unsuitable for distributing an unsigned test installer, while draft releases remain visible only to users with push access. This mode lets the maintainer verify and privately relay an exact-commit build without changing the public release or offpdf.com.

Validation

• actionlint .github/workflows/windows-release.yml
• extracted PowerShell step parses successfully with System.Management.Automation.Language.Parser
• git diff --check

Refs #14