fix(input): cursors follow the pointer while dragging and snap on drop

include/plot/input/controller.hpp

@@ -89,7 +89,10 @@ using plot::RegionGrab;
 ///     grabbed cursor's orientation owns -- `set_cursor_x()` for a marker,
 ///     `set_cursor_y()` for a ruler -- and a dragged ruler is clamped to its own y
 ///     axis' range, because a pointer past the top or bottom of `axes` has no data
-///     under it to follow (see `move_cursor()`).
+///     under it to follow (see `move_cursor()`). The writes are *free*, so the marker
+///     tracks the pointer for the whole gesture rather than hopping between the samples
+///     it would otherwise snap to; the cursor's own snap rule is applied once, on
+///     release (`Plot::snap_cursor()`), exactly as a region's is.
 ///  2. **Regions next**, resolved nearest-first against the geometry `Plot::layout()`
 ///     measured: a bound within the grab radius beats the body (so the bound of a
 ///     wide region is grabbable even though the pointer is inside the fill), the
@@ -161,7 +164,7 @@ using plot::RegionGrab;
 /// The plot geometry the controller hit-tests in is device pixels (see
 /// `PlotLayout`), and each adapter converts its pointer coordinates into the same
 /// space before forwarding them. A tolerance written as a device constant would
-/// therefore change its *physical* size with the display: a 5 px grab radius is
+/// therefore change its *physical* size with the display: a 7 px grab radius is
 /// half the hand movement on a 2x panel, so the same cursor becomes harder to
 /// catch there. Both tolerances -- the cursor grab radius and the box-zoom click
 /// threshold -- are therefore logical constants, converted through