fix(data): the gather origin is the caller's, so a deep zoom draws on the sample

`Series::gather()` stages x as f32 relative to `x0()` -- the start of the
*retained window* -- so the value's ULP was a fraction of data the view need
not even hold: 1.9 us over a 20 s ring. A readout quotes `x_at()` in exact
f64, so at a 10 us view across 700 px that put more than a hundred pixels
between where a sample is drawn and the x reported for it.

The origin is now a parameter, placed before `out_xy` so every call site is a
type error rather than a silent change of behaviour. `Renderer::draw_traces`
passes the visible range's minimum, which makes the error a fixed 2^-24 of the
span on screen -- 4e-5 of a pixel over a 700 px axis -- at every zoom.

`stage_xy()` loses its second origin as a consequence: the gather origin and
the draw origin are now always the same value, so the linear case hands the
gathered buffer straight to `add_polyline_xy()` with no per-point copy.

`Plot::crosshair_point()` names the query itself, the one origin that cannot
lose precision for a caller that might.

Guarded by `a_gather_origin_at_the_view_keeps_a_deep_zoom_on_the_sample_it_quotes`,
which measures the error in pixels both ways: under 0.05 px with the view's
minimum, over 10 px with `x0()`.

include/plot/data/xy_series.hpp

@@ -53,8 +53,8 @@ class XYSeries final : public Series {
   Range x_bounds() const override;
   Range y_bounds() const override;
   const MinMaxPyramid& pyramid() const override;
-  /// Same contract as `Series::gather`, with x emitted relative to `x0()`.
-  void gather(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
+  /// Same contract as `Series::gather`, with x emitted relative to `x_origin`.
+  void gather(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
               GatherMode mode = GatherMode::Envelope) const override;
 
   /// The sample's own x.