Initial commit: APT-DNF Bridge workspace

- Core crate: Minimal shell-out implementation
- Advanced crate: Pluggable backends and enhanced features
- Main crate: Re-exports core + optional advanced features
- Feature flags: Users choose complexity level
- Examples: Working demonstrations of both approaches
- Documentation: Clear README explaining the structure

Implements the refined two-crate approach with workspace + feature flags.

apt-dnf-bridge-core/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "apt-dnf-bridge-core"
+version.workspace = true
+edition.workspace = true
+description = "Minimal shell-based APT transaction API (DNF-like)"
+license.workspace = true
+repository.workspace = true
+keywords.workspace = true
+categories.workspace = true
+
+[dependencies]
+anyhow.workspace = true
+serde.workspace = true
+thiserror.workspace = true
+tokio.workspace = true