fix(input): refuse non-finite region bounds, keep narrow regions movable, republish hot handles

include/plot/plot/plot.hpp

@@ -161,6 +161,12 @@ class Plot {
   /// onto the sample grid as it is added -- exactly as `add_cursor()` places a
   /// snapping cursor -- so a region handed to a plot that already holds samples is on
   /// the grid from its first frame.
+  ///
+  /// A region whose bounds are not both finite is *refused*: nothing is added, and the
+  /// returned id names no region (`region()` reads a default for it and `region_count()`
+  /// does not count it). Storing one would be worse than useless, because a `Free` region
+  /// holding a bound that is not a number could never be dragged back -- every motion
+  /// re-derives both bounds from the stored ones.
   u32 add_region(const Region& region);
 
   // --- accessors --------------------------------------------------------
@@ -340,6 +346,11 @@ class Plot {
   /// bounds under the pointer and put them on the sample grid once, on release,
   /// instead of both ends sticking as the pointer moves. Also drops the region's
   /// cached statistics.
+  ///
+  /// A pair of bounds that is not both finite is *refused*: `std::min`/`std::max` would
+  /// propagate a NaN through both stored bounds and a finite partner would be lost with
+  /// it, so a rejected call changes nothing at all -- the interval stays as it was, and
+  /// so do the statistics cached for it.
   void set_region(u32 id, f64 x0, f64 x1);
 
   /// Pulls a snapping region's bounds onto the sample grid: onto the nearest sample