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 90.56%. Comparing base (e0ff281) to head (2ed76d3).
⚠️ Report is 93 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1130      +/-   ##
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+ Coverage    82.18%   90.56%   +8.38%     
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  Files          122      131       +9     
  Lines        16355    17554    +1199     
===========================================
+ Hits         13441    15898    +2457     
+ Misses        2914     1656    -1258     

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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

[Gui-FernandesBR]

Following up on my "the example plots don't really make sense" comment — I found why, and it is not the plotting code.

static_margin is only ever discretized over the burn:

self.static_margin.set_discrete(lower=0, upper=self.motor.burn_out_time, samples=200)

With extrapolation="constant", every query past burn_out_time returns the value frozen at the edge of that range. So for the whole coast, apogee and descent the curve is not a computed static margin at all — it is one number repeated, which is exactly the flat/implausible shape in the plots you posted.

For a flight-long quantity, use Flight.stability_margin, which is a function of Mach and time and is evaluated over the whole flight. If you really want the static margin specifically, it has to be re-discretized over the flight's own time span before plotting.

Two other notes:

• The workflow runs on this PR had been stuck in action_required and never actually executed, so this PR has never had real CI. I approved them, so you will get results now.
• Once the curve is right, please post the regenerated plots again so we can compare.

[Gui-FernandesBR]

no need to commit docs/static/pr_examples/cp_evolution_calisto_like.png file