  We present spatially-resolved echelle spectroscopy of an intervening
MgII-FeII-MgI absorption-line system detected at $z_{\rm abs}=0.73379$ toward
the giant gravitational arc PSZ1 G311.65-18.48. The absorbing gas is associated
to an inclined disk-like star-forming galaxy, whose major axis is aligned with
the two arc-segments reported here. We probe in absorption the galaxy's
extended disk continuously, at $\approx 3$ kpc sampling, from its inner region
out to $15\times$ the optical radius. We detect strong ($W_0^{2796}>0.3$ \r{A})
coherent absorption along $13$ independent positions at impact parameters
$D=0$--$29$ kpc on one side of the galaxy, and no absorption at $D=28$--$57$
kpc on the opposite side (all de-lensed distances at $z_{\rm abs}$). We show
that: (1) the gas distribution is anisotropic; (2) $W_0^{2796}$, $W_0^{2600}$,
$W_0^{2852}$, and the ratio $W_0^{2600}\!/W_0^{2796}$, all anti-correlate with
$D$; (3) the $W_0^{2796}$-$D$ relation is not cuspy and exhibits significantly
less scatter than the quasar-absorber statistics; (4) the absorbing gas is
co-rotating with the galaxy out to $D \lesssim 20$ kpc, resembling a `flat'
rotation curve, but at $D\gtrsim 20$ kpc velocities decline below the
expectations from a 3D disk-model extrapolated from the nebular [OII] emission.
These signatures constitute unambiguous evidence for rotating extra-planar
diffuse gas, possibly also undergoing enriched accretion at its edge. Arguably,
we are witnessing some of the long-sought processes of the baryon cycle in a
single distant galaxy expected to be representative of such phenomena.
