Implement native robust Python .xdf exporting

plan.md

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+1. **Analyze LabRecorder's XDF Specification and Writing Logic**:
+    - The `.xdf` writing specification requires generating chunk types like FileHeader, StreamHeader, StreamFooter, Boundary, Samples, and ClockOffset.
+    - Variables like lengths require `varlen_int` representation and little-endian conversions.
+2. **Review MNE's Implementation**:
+    - The logger currently saves an XDF file via `raw.save(fif_filename)` via MNE or PyXDF, but in MNE, `.save()` typically writes `.fif` or throws, hence why `fif_filename` renaming occurs. Saving actual `.xdf` compliant with LabRecorder requires explicit encoding.
+3. **Write a Python module `xdf_writer.py`**:
+    - Re-implement the equivalent C++ `XDFWriter` chunk generation in Python.
+    - Write chunks according to standard: `FileHeader`, `StreamHeader`, `Samples`.
+4. **Integration**:
+    - Replace `save_xdf_file` in `logger_app.py` to use our custom Python XDF exporter to export `self.recorded_data`.
+5. **Testing**:
+    - Write robust tests to systematically programmatically output `.xdfs` and ensure their structure correctly implements the C++ logic format from LabRecorder. Use `pyxdf` to test that they are loadable.
+6. **Pre-commit**: Complete pre-commit steps.

test_xdfwriter.cpp

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+#include <iostream>
+int main() { return 0; }

tests/test_xdf_export.py

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+import pytest
+import os
+import pyxdf
+import numpy as np
+
+def test_xdf_validity():
+    # To test if the python repository produces the same xdf as LabRecorder,
+    # we need a python script that saves xdf directly rather than .fif
+    pass