Add support for node-thermal-printer by solving issue with buffer ref not working with n-api

[andidev]

fixes #1

[ssxv]

Background

The Windows implementation had a critical N-API threading issue where Napi::Reference<Napi::Buffer<char>> was accessed from background threads, causing:

Fatal error in v8::HandleScope::CreateHandle()
Cannot create a handle without a HandleScope

Technical Comparison

Aspect	Windows (Before Patch)	Windows (After Patch)	POSIX (Current)
Small Buffers (≤4MB)	Buffer reference ❌	Copy to vector ✅	Copy to string ✅
Large Buffers (>4MB)	Temp file ✅	Temp file ✅	Temp file ✅
Thread Safety	HandleScope violation ❌	Safe ✅	Safe ✅
V8 Handle Access	Background thread ❌	None ✅	None ✅

Root Cause Analysis

The Windows implementation attempted a "zero-copy optimization" by keeping buffer references alive, but this violated N-API threading rules. V8 handles (including Napi::Reference) can only be accessed from the main thread within a proper HandleScope.

The POSIX implementation was designed correctly from the start - it never attempted buffer references and always used the safe copy-or-temp-file strategy.

Conclusion
✅ POSIX implementation requires NO changes - it already follows the safe pattern
✅ Windows patch correctly fixes the threading violation
✅ Both platforms now use identical safe strategies

The memory overhead of copying small buffers (≤4MB) is negligible compared to the benefit of crash-free operation, and large buffers still use efficient temp file streaming on both platforms.