Require `step: float` input to `slice_from_time()`

There's been way too many issues when trying to calculate this
dynamically from the input array, so just expect the caller to know what
it's doing and don't bother with ever hitting the error case of
calculating and incorrect value internally.

piker/data/_pathops.py

@@ -295,7 +295,7 @@ def slice_from_time(
     arr: np.ndarray,
     start_t: float,
     stop_t: float,
-    step: int | None = None,
+    step: float,  # sampler period step-diff
 
 ) -> slice:
     '''
@@ -324,12 +324,6 @@ def slice_from_time(
     # end of the input array.
     read_i_max = arr.shape[0]
 
-    # TODO: require this is always passed in?
-    if step is None:
-        step = round(t_last - times[-2])
-        if step == 0:
-            step = 1
-
     # compute (presumed) uniform-time-step index offsets
     i_start_t = floor(start_t)
     read_i_start = floor(((i_start_t - t_first) // step)) - 1
@@ -412,7 +406,7 @@ def slice_from_time(
             times[read_i_start:],
             # times,
             i_stop_t,
-            side='left',
+            side='right',
         )
 
         if (