Add module unregistration to buswatch-sdk

[lowhung]

Fixes #20

Summary

Adds module unregistration functionality to buswatch-sdk, enabling clean lifecycle management for temporary or dynamic modules.

Changes

• Added Instrumentor::unregister(name) method to remove modules from internal state
• Added GlobalState::unregister_module(name) for module cleanup
• Returns true if module was found and removed, false otherwise
• Updated README.md with comprehensive unregistration documentation
• Updated CHANGELOG.md

Tests

Added comprehensive test coverage:

• ✅ Unregister existing module
• ✅ Unregister non-existent module
• ✅ Verify metrics are removed after unregister
• ✅ Verify re-registering works after unregister (fresh state)
• ✅ Old handles still work but don't appear in snapshots
• ✅ Unregistering one module doesn't affect others
• ✅ Multiple unregister calls are safe

All 54 tests pass successfully.

Use Cases

• Temporary worker modules that complete their tasks
• Dynamic module registration/unregistration in multi-tenant systems
• Cleanup of inactive modules to reduce memory footprint
• Testing scenarios requiring module isolation