  Placing a nanomechanical object in the evanescent near-field of a high-$Q$
optical microcavity gives access to strong gradient forces and
quantum-noise-limited displacement readout, offering an attractive platform for
precision sensing technology and basic quantum optics research. Robustly
implementing this platform is challenging, however, as it requires separating
optically smooth surfaces by $\lesssim\lambda/10$. Here we describe a
fully-integrated evanescent opto-nanomechanical transducer based on a
high-stress Si$_3$N$_4$ nanobeam monolithically suspended above a SiO$_2$
microdisk cavity. Employing a novel vertical integration technique based on
planarized sacrificial layers, we achieve beam-disk gaps as little as 25 nm
while maintaining mechanical $Q\times f>10^{12}$ Hz and intrinsic optical
$Q\sim10^7$. The combined low loss, small gap, and parallel-plane geometry
result in exceptionally efficient transduction, characterizing by
radio-frequency flexural modes with vacuum optomechanical coupling rates of 100
kHz, single-photon cooperativities in excess of unity, and zero-point frequency
(displacement) noise amplitudes of 10 kHz (fm)/$\surd$Hz. In conjunction with
the high power handling capacity of SiO$_2$ and low extraneous substrate noise,
the transducer operates particularly well as a sensor. Deploying it in a 4 K
cryostat, we recently demonstrated a displacement imprecision 40 dB below that
at the standard quantum limit (SQL) with an imprecision-back-action product
$<5\cdot\hbar$. In this report we provide a comprehensive description of device
design, fabrication, and characterization, with an emphasis on extending
Heisenberg-limited readout to room temperature. Towards this end, we describe a
room temperature experiment in which a displacement imprecision 30 dB below
that at the SQL and an imprecision-back-action product $<75\cdot\hbar$ is
achieved.
