PVG Ascents with fixed burntimes and multiple unguided upper stages do not enter TERMINAL guidance on the booster

[lamont-granquist]

If there is an "optimized" free-burntime booster stage, then it will correctly enter TERMINAL guidance if all the upper stages are unguided. But if we're doing a fixed burntime max-energy ascent, then guidance never enters TERMINAL state, even though we're about to lose all our degrees of freedom.

PVG handles this reasonably well, but there appears to be a control kick right before the upper stage separates. The new guidance in #2007 has much worse behavior with this bug.

[lamont-granquist]

(fixed the worse behavior with the new guidance, that was around the Optimizer not setting u0 on the phase, which is a bit of jank that probably needs to go away -- this still should get fixed though)