BUG: pd.to_datetime with low-reso unit and high-reso origin

[jbrockmendel]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

ts = pd.Timestamp("2016-01-01 00:00:00.000001")
dti = pd.to_datetime([1, 2, 3], unit="D", origin=ts)

>>> dti
DatetimeIndex(['2016-01-02', '2016-01-03', '2016-01-04'], dtype='datetime64[ns]', freq=None)

Issue Description

@jorisvandenbossche pointed this out in #63347, but it turns out to be a problem on main too.

Expected Behavior

dti
DatetimeIndex(['2016-01-02 00:00:00.01', '2016-01-03 00:00:00.01', '2016-01-04 00:00:00.01'], dtype='datetime64[ns]', freq=None)

Installed Versions

Details

Replace this line with the output of pd.show_versions()

[jorisvandenbossche]

This is also the behaviour on released 2.x. It seems the origin gets rounded/truncated at the specified unit of numeric input (which also makes somewhat sense? but maybe it should not happen silently)