  We present a framework for high-redshift ($z \geq 7$) galaxy formation that
traces their dark matter (DM) and baryonic assembly in four cosmologies: Cold
Dark Matter (CDM) and Warm Dark Matter (WDM) with particle masses of $m_x =$
1.5, 3 and 5 ${\rm keV}$. We use the same astrophysical parameters regulating
star formation and feedback, chosen to match current observations of the
evolving ultra violet luminosity function (UV LF). We find that the assembly of
observable (with current and upcoming instruments) galaxies in CDM and $m_x
\geq 3 {\rm keV}$ WDM results in similar halo mass to light ratios (M/L),
stellar mass densities (SMDs) and UV LFs. However the suppression of
small-scale structure leads to a notably delayed and subsequently more rapid
stellar assembly in the $1.5 {\rm keV}$ WDM model. Thus galaxy assembly in $m_x
\leq 2 {\rm keV}$ WDM cosmologies is characterized by: (i) a dearth of
small-mass halos hosting faint galaxies; and (ii) a younger, more UV bright
stellar population, for a given stellar mass. The higher M/L ratio (effect ii)
partially compensates for the dearth of small-mass halos (effect i), making the
resulting UV LFs closer to CDM than expected from simple estimates of halo
abundances. We find that the redshift evolution of the SMD is a powerful probe
of the nature of DM. Integrating down to a limit of $M_{UV} =-16.5$ for the
James Webb Space Telescope (JWST), the SMD evolves as $\log$(SMD)$\propto -0.63
(1+z)$ in $m_x = 1.5 {\rm keV}$ WDM, as compared to $\log$(SMD)$\propto -0.44
(1+z)$ in CDM. Thus high-redshift stellar assembly provides a powerful testbed
for WDM models, accessible with the upcoming JWST.
