Fix CI: add Cargo.lock and missing x64-mingw32 platform

- Ensure Cargo.lock is tracked for reproducible cross-compilation builds
- Add x64-mingw32 to Rakefile cross_platform list to match CI workflow

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

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

Author: michaelklishin

Cargo.lock

@@ -15,6 +15,7 @@ RbSys::ExtensionTask.new("amq_protocol_native", GEMSPEC) do |ext|
     aarch64-linux-musl
     arm64-darwin
     x64-mingw-ucrt
+    x64-mingw32
     x86_64-darwin
     x86_64-linux
     x86_64-linux-musl