Minor issues: unused variables B/M/z, and observed flag not saved in freeze()

[chenglibo123]

Hi, thank you for the great OC‑SORT implementation!

While studying the C++ code, I noticed a few minor points that might be worth clarifying or improving:

1. Unused matrices B and M
   In KalmanFilterNew, both B and M are defined, initialized, and saved/restored in freeze()/unfreeze(), but they are never used in any actual computation (no control input, noise uncorrelated). If they are kept for completeness, a brief comment would be helpful; otherwise they could be removed to reduce clutter.

2. Unused member variable z (last observation)
   z is assigned and saved/restored, but it never participates in filter calculations (the innovation uses the passed pointer *z_ directly). It seems to serve only as an archive. Consider removing it or adding a comment explaining its purpose.

3. observed flag not saved in freeze()
   The observed boolean is not backed up in attr_saved during freeze(). For completeness and to make unfreeze() self‑contained, adding attr_saved.observed = observed; in freeze() and restoring it in unfreeze() would be nice (even though update() sets it to true right after unfreeze() – it doesn't hurt).

None of these affect algorithm correctness, but addressing them would make the code cleaner.