crosshair: axis badges, clustered cursor labels, and the shared-y horizontal line

- Axis badges: the y value in the y label strip and the x time in the x strip, each a
  full-opacity panel in the crosshair's colour, **centred on the line it names**. The tick
  text is flushed before the fill -- the over-text batch is flushed after every line batch,
  so without that flush the label behind printed through the panel (the "transparent
  backplate" defect).
- Cursor labels: collected across the cursor loop and clustered at its end, so two marks in
  one column share a panel with one line each instead of overprinting, and a label stays on
  its own mark's row.
- Figure: a split row's cells share one y (`set_share_row_y`), so the horizontal line is
  drawn in every cell of the hovered row -- one value, one scale, spanning the row -- and
  still in no other row.

Tests: `figure_crosshair_horizontal_spans_a_split_rows_cells`, plus the render-smoke
additions for the badge placement/flush and the clustered labels.

include/plot/render/renderer.hpp

@@ -95,6 +95,24 @@ struct FrameStats {
 /// a guard -- and the frame reports it (see `last_frame_had_gl_error()`).
 ///
 /// Not thread-safe, and confined to the thread that owns the current context.
+
+/// One cursor label's measured text and placed badge: what `Renderer::collect_cursor_label()`
+/// builds and `Renderer::draw_clustered_cursor_labels()` places and draws. A rendering
+/// detail of the cursor labels' backplates, declared at namespace scope only because a
+/// private nested type cannot be named in an out-of-class member definition's signature.
+struct CursorLabel {
+  std::string text;  ///< the one line the backplate carries
+  Color border{};    ///< the panel's border: the cursor's own colour
+  f32 anchor = 0.0f; ///< the coordinate the label belongs to (x for a marker, y for a ruler)
+  f32 w = 0.0f;      ///< the measured text width
+  bool horizontal = false;
+  /// The placed pen and backplate: centred on the anchor across the mark, nudged onto a
+  /// free row when a close neighbour's badge is in the way, and clustered into one panel
+  /// by `draw_clustered_cursor_labels()`.
+  Vec2 pen{};
+  Rect box{};
+};
+
 class Renderer {
  public:
   Renderer() noexcept = default;
@@ -621,13 +639,22 @@ class Renderer {
   /// the clip, and a range maximum that undershoots the edge by a ulp is not mistaken for
   /// off the plot.
   void draw_cursors(const Plot& plot, const PlotLayout& layout);
-  /// One cursor's label: the caller's caption for either orientation, and for a horizontal
-  /// ruler the value itself, written in its y axis' own tick format (`Axis::format_value()`)
-  /// -- the number a ruler exists to be read off. A vertical cursor's caption starts at the
-  /// top of the axes beside its marker; a ruler's text starts at the axes' left edge on its
-  /// own row, with the caption first on that line when there is one.
-  void draw_cursor_label(const Plot& plot, const PlotLayout& layout, const Cursor& cursor,
-                         f32 at);
+  /// One cursor's label, measured rather than drawn: a horizontal ruler's line is the value
+  /// itself, written in its y axis' own tick format (`Axis::format_value()`) -- the number a
+  /// ruler exists to be read off -- with the caption first on the line when there is one; a
+  /// vertical cursor's line is its caption, beside the marker at the top of the axes. The
+  /// label is then placed (centred on its mark, stacked clear of close neighbours) and drawn
+  /// clustered (`draw_clustered_cursor_labels()`), which is what keeps a pair of readout
+  /// marks half a division apart readable: two texts over one column is a smear, one panel
+  /// with two lines is a readout.
+  void collect_cursor_label(const Plot& plot, const PlotLayout& layout, const Cursor& cursor,
+                            f32 at, std::vector<CursorLabel>& labels);
+  /// Draws the placed labels' backplates -- one panel per cluster of meeting boxes, every
+  /// line at its own placed pen -- and stages the lines. Clustering is what keeps a pair of
+  /// readout marks half a division apart readable: two texts over one column is a smear, one
+  /// panel with two lines is a readout.
+  void draw_clustered_cursor_labels(const PlotLayout& layout,
+                                    const std::vector<CursorLabel>& labels);
   /// The regions: the fill, one bound per edge, the optional label. Every rect comes
   /// from `PlotLayout::regions` -- the same geometry the controller hit-tests -- so
   /// a region is drawn where it can be grabbed, and neither side derives the other's
@@ -656,8 +683,12 @@ class Renderer {
   /// outermost pixels the fill covers (the crisp-hairline rule -- a border drawn on the measured
   /// rect's fractional edges is lit half-heartedly, and a frame bright on three sides and dim on the
   /// fourth reads as a mistake). One helper, two callers: the border's colour is the only thing a
-  /// legend panel and a readout panel do not share.
-  void draw_readout_panel(const Rect& panel, Color border);
+  /// legend panel and a readout panel do not share. `background_alpha` scales the frame's own
+  /// background under the border: the legend and the crosshair's readout keep the shared
+  /// translucent backplate (what they cover stays faintly visible), while a *badge* -- a
+  /// cursor's value box, the crosshair's axis badges -- is read against whatever it sits on
+  /// (ticks, a noisy trace) and is drawn opaque.
+  void draw_readout_panel(const Rect& panel, Color border, float background_alpha);
   /// One trace's colour swatch in `box`, opaque whatever the trace's own alpha is: a swatch drawn at
   /// a trace's 15% would be the least legible thing in the frame, and the swatch is the one element
   /// that says which colour an entry or a readout row *is*. The legend's entries and the crosshair's
@@ -687,6 +718,12 @@ class Renderer {
   /// Drawn after every other interior pass -- the legend included -- because it is what the
   /// pointer is on: a readout the legend covered would be the feature failing at its one job.
   void draw_crosshair_readout(const Plot& plot, const PlotLayout& layout);
+  /// The crosshair's **axis badges** (`CrosshairLayout::y_axis_badge`/`x_axis_badge`): the
+  /// values the two lines meet the axes at, each on the shared readout backplate with its
+  /// border in the crosshair's colour, drawn over the tick labels they sit among. Called
+  /// from `draw_plot()` after the label strips are drawn, which is the only order in which a
+  /// badge in a strip can sit on the ticks and still be read.
+  void draw_crosshair_axis_badges(const PlotLayout& layout);
 
   /// The host's box-zoom frame (`PlotLayout::selection`): a fill at `selection_fill` under a border at
   /// `selection_edge`, or nothing at all when nothing was swept. Drawn last of the interior passes, so