  We show how to absorb fermionic quantum simulation's expensive
fermion-to-qubit mapping overhead into the overhead already incurred by
surface-code-based fault-tolerant quantum computing. The key idea is to process
information in surface-code twist defects, which behave like logical Majorana
fermions. Our approach encodes Dirac fermions, a key data type for simulation
applications, directly into logical Majorana fermions rather than atop a
logical qubit layer in the architecture. Using quantum simulation of the
$N$-fermion 2D Fermi-Hubbard model as an exemplar, we demonstrate two immediate
algorithmic improvements. First, by preserving the model's locality at the
logical level, we reduce the asymptotic Trotter-Suzuki quantum circuit depth
from $\mathcal{O}(\sqrt{N})$ in a typical Jordan-Wigner encoding to
$\mathcal{O}(1)$ in our encoding. Second, by exploiting optimizations manifest
for logical fermions but less obvious for logical qubits, we reduce the
$T$-count of the block-encoding \textsc{select} oracle by 20\% over standard
implementations, even when realized by logical qubits and not logical fermions.
