  We prove convergence of the 2- and 4-point fermionic observables of the
FK-Ising model on simply connected domains discretised by a planar isoradial
lattice in massive (near-critical) scaling limit. The former is alternatively
known as a (fermionic) martingale observable (MO) for the massive interface,
and in particular encapsulates boundary visit probabilties of the interface.
The latter encodes connection probabilities in the 4-point alternating
(generalised Dobrushin) boundary condition, whose exact convergence is then
further analysed to yield crossing estimates for general boundary conditions.
These observables satisfy a massive version of s-holomorphicity [Smi10], and we
develop robust strategies to exploit this condition which do not require any
regularity assumption of the domain or a particular direction of perturbation.
