Prepare 3.0.0 and restore full PR checks

[Redrrx]

Changes

• Preserve version 3.0.0 and the existing application and test code.
• Run Rust formatting, Clippy, Python syntax, and all six platform builds on PRs.
• Run Linux decryption tests against the already-built release executable.
• Do not build again on branch pushes or merges into main.
• Publish only manually pushed version tags matching Cargo.toml and Cargo.lock on main.
• Include commits since the previous version tag plus GitHub-generated release notes.

Local validation

Rust formatting, strict Clippy, native release 3.0.0 build, workflow validation, and all four unchanged E2E tests passed.

Release

Merge after CI passes. The owner will create and push v3.0.0 from updated main; no release tag has been created.