  We present 0.075 (~400 pc) resolution ALMA observations of the [CII] and dust
continuum emission from the host galaxy of the z=6.5406 quasar, P036+03. We
find that the emission arises from a thin, rotating disk with an effective
radius of 0.21" (1.1 kpc). The velocity dispersion of the disk is consistent
with a constant value of 66.4+-1.0 km/s, yielding a scale height of 80+-30 pc.
The [CII] velocity field reveals a distortion that we attribute to a warp in
the disk. Modeling this warped disk yields an inclination estimate of 40.4+-1.3
degrees and a rotational velocity of 116+-3 km/s. The resulting dynamical mass
estimate of (1.96+-0.10) x 10^10 Msun is lower than previous estimates, which
strengthens the conclusion that the host galaxy is less massive than expected
based on local scaling relations between the black hole mass and the host
galaxy mass. Using archival MUSE Ly-alpha observations, we argue that
counter-rotating halo gas could provide the torque needed to warp the disk. We
further detect a region with excess (15-sigma) dust continuum emission, which
is located 1.3 kpc northwest of the galaxy's center and is gravitationally
unstable (Toomre-Q < 0.04). We posit this is a star-forming region whose
formation was triggered by the warp, because the region is located within a
part of the warped disk where gas can efficiently lose angular momentum. The
combined ALMA and MUSE imaging provides a unique view of how gas interactions
within the disk-halo interface can influence the growth of massive galaxies
within the first billion years of the universe.
