ENH: plot flight center-of-pressure evolution (#954)

[thatrandomasiandev]

Summary

• Add Flight.plots.center_of_pressure() to plot CP position vs time, evaluated at the flight Mach number at each step
• Overlay center of mass on the same axis and static margin on a twin axis, with out-of-rail / burnout / apogee markers
• Wire the new plot into Flight.plots.all() and cover show/save paths in unit tests

Fixes #954

Test plan

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 MPLBACKEND=Agg python -m pytest tests/unit/test_plots.py::test_flight_center_of_pressure_plot -q
• Manual: flight.plots.center_of_pressure() on a Calisto flight shows CP/CM/static-margin curves through first event time

[Gui-FernandesBR]

we need to see one or two examples of the new plot before merging it please

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.54%. Comparing base (e0ff281) to head (28e496c).
⚠️ Report is 43 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1130      +/-   ##
===========================================
+ Coverage    82.18%   82.54%   +0.36%     
===========================================
  Files          122      128       +6     
  Lines        16355    16644     +289     
===========================================
+ Hits         13441    13739     +298     
+ Misses        2914     2905       -9     

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[thatrandomasiandev]

@Gui-FernandesBR here are two examples from this branch:

1. Full-ish ascent (rail → burnout → apogee window), truncated at first_event_time like other stability plots:
   

2. terminate_on_apogee=True:
   

Both generated with flight.plots.center_of_pressure(filename=...) under MPLBACKEND=Agg. Happy to tweak axis limits / twin-axis layout if you want something closer to the existing stability plots.

[Gui-FernandesBR]

the example plots don't really make sense