  We present the first Faraday rotation measure (RM) grid study of an
individual low-mass cluster -- the Fornax cluster -- which is presently
undergoing a series of mergers. Exploiting commissioning data for the
POlarisation Sky Survey of the Universe's Magnetism (POSSUM) covering a
$\sim34$ square degree sky area using the Australian Square Kilometre Array
Pathfinder (ASKAP), we achieve an RM grid density of $\sim25$ RMs per square
degree from a 280 MHz band centred at 887 MHz, which is similar to expectations
for forthcoming GHz-frequency all-sky surveys. We thereby probe the extended
magnetoionic structure of the cluster in unprecedented detail. We find that the
scatter in the Faraday RM of confirmed background sources is increased by
$16.8\pm2.4$ rad m$^{-2}$ within 1 degree (360 kpc) projected distance to the
cluster centre, which is 2--4 times more extended than the presently-detectable
X-ray-emitting intracluster medium (ICM). The Faraday-active plasma is more
massive than the X-ray-emitting ICM, with an average density that broadly
matches expectations for the Warm-Hot Intergalactic Medium. The morphology of
the Faraday depth enhancement exhibits the classic morphology of an
astrophysical bow shock on the southwest side of the main Fornax cluster, and
an extended, swept-back wake on the northeastern side. Our favoured explanation
is an ongoing merger between the main cluster and a sub-cluster to the
southwest. The shock's Mach angle and stand-off distance lead to a
self-consistent transonic merger speed with Mach 1.06. The region hosting the
Faraday depth enhancement shows a decrement in both total and polarised
intensity. We fail to identify a satisfactory explanation for this; further
observations are warranted. Generally, our study illustrates the scientific
returns that can be expected from all-sky grids of discrete sources generated
by forthcoming all-sky radio surveys.
