feat(plot): a figure can keep an empty band to the right of its data

`layout_.axes` reached the viewport's right edge, so the rightmost x tick
label -- centred on its tick -- had half of itself outside the plot, and
the newest sample's ink landed on the frame. `Plot::set_right_gutter()`
reserves a band there, out of the same `1 - kMinAxesShare` allowance the
label and title strips are served from, so it can no more squeeze `axes`
below the half-viewport floor than they can. `Figure` sets one value on
every row, the left gutter's reason: the rows' axes have to end on a
common column as well as start on one.

Nothing is drawn in the band, so unlike the strips it is a want and never
a measurement: no measuring pass, and it cannot feed the layout's fixed
point.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/plot/figure.cpp

@@ -451,6 +451,10 @@ const FigureLayout& Figure::layout() {
     // not drawn once per row. A figure told to label every row keeps them everywhere,
     // and then there is no shared axis to speak of: each row has its own.
     p.set_x_strip_mode(labels_bottom_only_ && row != bottom_row ? XStrip::None : XStrip::Both);
+    // Before either pass below, and the same value for every row: nothing is measured on
+    // the right, so unlike the left gutter this needs no measurement pass -- the rows end
+    // on a common column because they are all told the one width.
+    p.set_right_gutter(right_gutter_);
   }
   // After the viewports, so a row's ticks and grid -- built from its axes rect -- and
   // its transforms describe this frame's rects; and before the rows lay themselves

src/plot/plot.cpp

@@ -1039,12 +1039,21 @@ const PlotLayout& Plot::layout() {
     x_height = std::min(x_height, x_budget);
     const f32 y_title_width = std::min(y_title_w, y_budget - y_width);
     const f32 x_title_height = std::min(x_title_h, x_budget - x_height);
+    // The right-hand band, out of what is left of the same allowance the two left
+    // strips were served from -- so the three together cannot take more than
+    // `1 - kMinAxesShare` of the width however much each asks for. Nothing is drawn
+    // in it, so unlike the strips above it is a want and never a measurement, and it
+    // cannot feed the fixed point below: the y labels are chosen from the row's range
+    // and height, so a narrower `axes` does not change what they need.
+    const f32 right_gutter =
+        std::min(right_gutter_, y_budget - (y_width + y_title_width));
     layout_.y_axis_width = y_width;
     layout_.x_axis_height = x_height;
     layout_.y_title_width = y_title_width;
     layout_.x_title_height = x_title_height;
+    layout_.right_gutter = right_gutter;
     layout_.axes = Rect{viewport_.x + y_width + y_title_width, viewport_.y,
-                        vw - (y_width + y_title_width),
+                        vw - (y_width + y_title_width + right_gutter),
                         vh - (x_height + x_title_height)};
     layout_.y_title_area = Rect{viewport_.x, layout_.axes.y, y_title_width, layout_.axes.h};
     layout_.y_label_area =