  We present an overview and first results from a $M$-band spectroscopic survey
of planet-forming disks performed with iSHELL on IRTF, using two slits that
provide resolving power R $\approx$ 60,000-92,000 (5-3.3 km/s). iSHELL provides
a nearly complete coverage at 4.52-5.24 $\mu$m in one shot, covering $>50$
lines from the R and P branches of $^{12}$CO and $^{13}$CO for each of multiple
vibrational levels, and providing unprecedented information on the excitation
of multiple emission and absorption components. Some of the most notable new
findings of this survey are: 1) the detection of two CO Keplerian rings at $<2$
au (in HD 259431), 2) the detection of H${_2}$O ro-vibrational lines at 5
$\mu$m (in AS 205 N), and 3) the common kinematic variability of CO lines over
timescales of 1-14 years. By homogeneously analyzing this survey together with
a previous VLT-CRIRES survey of cooler stars, we discuss a unified view of CO
spectra where emission and absorption components scan the disk surface across
radii from a dust-free region within dust sublimation out to $\approx10$ au. We
classify two fundamental types of CO line shapes interpreted as emission from
Keplerian rings (double-peak lines) and a disk surface plus a low-velocity part
of a wind (triangular lines), where CO excitation reflects different emitting
regions (and their gas-to-dust ratio) rather than just the irradiation
spectrum. A disk+wind interpretation for the triangular lines naturally
explains several properties observed in CO spectra, including the line
blue-shifts, line shapes that turn into narrow absorption at high inclinations,
and the frequency of disk winds as a function of stellar type.
