  Fast material ejected dynamically over $<10$ ms during the merger of a binary
neutron-star (BNS) system can give rise to distinctive electromagnetic
counterparts to the system's gravitational-wave emission that can serve as a
"smoking gun" to distinguish between a BNS and a NS-black-hole merger. We
present novel ab-initio modeling of the associated kilonova precursor and
kilonova afterglow based on three-dimensional general-relativistic
magneto-hydrodynamic simulations of BNS mergers with tabulated,
composition-dependent, finite-temperature equations of state (EOSs), weak
interactions, and approximate neutrino transport. We analyze dynamical mass
ejection from 1.35-1.35Msun binaries, typical of Galactic double-NS systems and
consistent with properties of the first observed BNS merger GW170817, using
three nuclear EOSs that span the range of allowed compactness. Nuclear reaction
network calculations yield a robust 2nd-to-3rd-peak r-process. We find few
1e-6Msun of fast ($v>0.6$c) ejecta that give rise to broad-band synchrotron
emission on ~yr timescales, consistent with recent tentative evidence for
excess X-ray/radio emission following GW170817. We find 2e-5Msun of free
neutrons that power a kilonova precursor on <h timescale. A boost in early
UV/optical brightness by a factor of a few due to previously neglected
relativistic effects, with appreciable enhancements up to 10h post-merger,
provides promising prospects for future detection with UV/optical telescopes
such as Swift or ULTRASAT out to 250Mpc. We find that a recently predicted
opacity boost due to highly ionized lanthanides at ~70000K is unlikely to
affect the early kilonova lightcurve based on the obtained ejecta structures.
Azimuthal inhomogeneities in dynamical ejecta composition for soft EOSs found
here ("lanthanide/actinide pockets") may have observable consequences for both
early kilonova and late-time nebular emission.
