Consolidate provisional thickness and tracer calculations in tracer advection #649

Tracer advection computes the provisional thickness and tracers (after horizontal advection only) in several places. As @mwarusz and @brian-oneill noted in review of #632, these should be computed once and shared by horizontal and vertical FCT.

• Horizontal FCT's HProv (TendencyTerms.h) is the same thickness as ProvPseudoThickness (PseudoThicknessAuxVars.h), from the same inputs. Both limiters also compute the thickness after vertical transport (HNewInv there, InvNewProvThick in VertAdv.cpp).
• Horizontal FCT already computes the provisional tracer in TracerCur (TendencyTerms.h), one tracer at a time. Start vertical FCT from the tracer after horizontal advection #632 adds ProvTracers in Tendencies.cpp to compute it again for vertical FCT.

Sharing the provisional tracer needs a tracer dimension on TracerCur, and the non-FCT horizontal advection would have to compute it too, since vertical FCT can run with horizontal FCT off. Both cases need some thought, so this was left out of #632.

Posted by Claude Code on @xylar's behalf. The analysis and wording above are AI-authored; please check them accordingly.