fix: Track Cargo.lock for reproducible builds and Docker

Binary projects should commit Cargo.lock to ensure reproducible builds.
This fixes the Docker build which requires Cargo.lock.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: sionsmith

.gitignore

@@ -1,7 +1,6 @@
 # Rust build artifacts
 /target/
 **/*.rs.bk
-Cargo.lock
 
 # IDE
 .idea/