fix: add mutex to prevent concurrent CGO access crashes

[wayne-o]

Summary

• Add sync.Mutex to serialize all exported CGO functions
• Prevents SIGSEGV crashes when the shared library is called concurrently from multiple threads

Problem

When using the compiled shared library (.so/.dylib) from .NET via P/Invoke, concurrent calls to exported functions cause segmentation faults (exit code 139). This happens because:

1. The global chainId variable is written without synchronization
2. The client.GetClient() / client.CreateClient() functions use a non-thread-safe map internally

Solution

Wrap all 19 exported functions with mu.Lock() / defer mu.Unlock() to ensure only one thread can access the Go runtime state at a time.

Testing

Tested with concurrent .NET threads (20 threads × 10 iterations = 200 concurrent signing operations) - all pass without crashes.

[alexvelea]

The global chainId variable is written without synchronization

This is true. I'm gonna fix this.

The client.GetClient() / client.CreateClient() functions use a non-thread-safe map internally

This is not true, all the access to maps / defaults are guarded by txClientMu

Wrapping all methods in a Mutex is not desired, as signing takes around 2ms and signing in paralel should be supported

[alexvelea]

Can you check if it works in #43 @wayne-o ?
I've added tmp-build to the repo.

Also, if you could provide some stack traces, highly welcomed.