harden binary parser against integer underflow and unsafe type punning

[SABITHSAHEB]

This patch improves the safety of the binary STL loader when handling malformed or truncated files.

Changes

• Rejects STL files that are too small to contain a valid header.
• Handles ftell() failures and adds a missing fopen() null check.
• Warns when the triangle count stored in the header does not match the file size.
• Replaces raw float pointer casting with a safer memcpy-based implementation.
• Replaces temporary C-style arrays with std::array.

These changes do not affect valid STL files and preserve existing parsing behavior.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Testing

Added tests for:

• Truncated STL files.
• Header triangle count mismatches.
• Normal STL loading (testModel.stl, 564 triangles).