fix: Windows build support and ONNX export from gated checkpoint

[TONiiV]

Summary

• Add -allow-unsupported-compiler to CMakeLists.txt for CUDA/MSVC 19.50+ compatibility
• Add SAM3_API dllexport macro to SAM3TrackerBoxInput and SAM3TrackerPointsInput (fixes LNK2019 unresolved externals on Windows)
• Add scripts/load_sam3.py shared helper to load sub-models from the combined Sam3VideoModel checkpoint with correct weight remapping
• Update all export scripts to use load_sam3 helpers; switch PCS export to dynamo=True (fixes tracing failures with newer transformers)

Notes

• No impact on Linux/Jetson builds (-allow-unsupported-compiler is a no-op when the compiler is already supported; SAM3_API macro is empty on non-Windows)
• strict=False in load_state_dict is intentional — some config keys differ between video model and standalone sub-models