[QL-10.x][EPIC-10][DEFER] SABR/SmileSection variants: shifted/FlochKennedy SABR, generic XABR, InterpolatedSmileSection, FdSabr

[benbenbang]

SABR + SmileSection: deferred variants

Deferred visibly from B3a (#582-585), which ships the core SABR formula + calibration + the SmileSection
trait + SabrSmileSection (the foundation for the B3b swaption vol cube).

Deferred

• shiftedSabrVolatility (sabr.hpp:84) - the displaced-diffusion SABR (shift != 0); and the
  shift != 0 path on SabrSmileSection.
• sabrFlochKennedyVolatility (sabr.hpp:94) - the Le Floch-Kennedy expansion.
• The generic XABRInterpolation<Specs> framework - B3a ports SABRInterpolation as a concrete
  calibration; the generic template + other smile models (SVI, Vanna-Volga, ZABR) are out of scope.
• InterpolatedSmileSection and the SmileSectionUtils.
• SmileSection::density / vega if not ported in T3.
• FdSabr (the PDE SABR model, fdsabr.cpp).
• SABRInterpolation robustness knobs (errorAccept / useMaxError / dontThrow) if deferred in T2.

Priority: low-medium (the core SABR + smile + calibration are the load-bearing pieces for the cube).
Epic: EPIC-10 #358.