Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Every aerodynamic surface is now rooted in GenericSurface and stores its
force coefficients in the body frame (cN/cY/cA) plus the cm/cn/cl moments,
exposing all nine coefficients (cL/cD/cQ/cN/cY/cA/cm/cn/cl) with the wind
trio lazily derived. A force_convention argument lets users supply wind- or
body-frame coefficients. Barrowman surfaces (nose, tail, fin sets) keep the
classic geometric normal-force/moment method, report the force at the
geometric center of pressure via the classic 180-degree surface rotation,
and expose cN_alpha/cY_beta stability slopes (the old clalpha relabelled).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Replace the poorly defined calisto_linear_generic fixture, which kept the
Barrowman nose cone and tail and swapped only the fins for a
LinearGenericSurface with arbitrary made-up coefficients. The new fixture is a
standalone Calisto whose nose cone, tail and fins are all LinearGenericSurfaces
built from coefficient curves extracted off the standard Barrowman surfaces
(normal-force-curve slope, center of pressure, fin roll damping).

Each linear surface applies its force at its own origin and is placed at the
source surface's center-of-pressure station, so both the static-margin path and
the flight-moment path land at the same point as calisto_robust. The resulting
flight matches the standard Calisto (identical apogee, out-of-rail time and
ascent angle of attack), so the fixture now exercises the linear
generic-surface path against a known-good reference.

Also fix test_linear_generic_surface_flight_is_stable to check the angle of
attack only during the ascent off the rail: on the rail the freestream speed is
~0 and the angle of attack is reported as a degenerate 90 degrees for any
launcher, which previously failed the < 45 assertion.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>