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/array_series.hpp

@@ -33,7 +33,7 @@ class ArraySeries final : public Series {
   Range x_bounds() const override;
   Range y_bounds() const override;
   const MinMaxPyramid& pyramid() const override;
-  void gather(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
+  void gather(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
               GatherMode mode = GatherMode::Envelope) const override;
 
  protected:

include/plot/data/series.hpp

@@ -59,12 +59,20 @@ class Series {
 
   /// Appends interleaved (x, y) pairs for level `level`.
   ///
-  /// **x is relative to `x0()`**: the emitted value is `x_at(i) - x0()` as
-  /// f32, never the absolute coordinate. A consumer reconstructs the absolute
-  /// x in f64 (`x0() + emitted`) before mapping it to the screen; staging an
-  /// absolute epoch-time x as f32 would quantise it to a grid coarser than
-  /// the pixel (f32 carries 24 bits -- 1 s at 1.7e9), while the relative
-  /// value `i*dx` is small and exact. y is unchanged.
+  /// **x is relative to `x_origin`**: the emitted value is `x_at(i) - x_origin` as f32, never
+  /// the absolute coordinate. A consumer reconstructs the absolute x in f64
+  /// (`x_origin + emitted`) before mapping it to the screen; staging an absolute epoch-time x
+  /// as f32 would quantise it to a grid coarser than the pixel (f32 carries 24 bits -- 1 s at
+  /// 1.7e9). y is unchanged.
+  ///
+  /// **The caller names the origin because only the caller knows the span the f32 has to be
+  /// exact against.** The origin was `x0()` -- the start of the series' *retained window* --
+  /// and a relative value's ULP is then a fraction of the whole window however little of it is
+  /// on screen: 1.9 us over 20 s of retained data, which at a view 10 us wide is more than a
+  /// hundred pixels between where a sample is *drawn* and the x a readout quotes for the same
+  /// sample (`x_at()`, exact f64). Passing the visible range's minimum instead makes the error
+  /// 2^-24 of the *visible span* -- a fixed 4e-5 of a pixel over a 700 px axis -- at every
+  /// zoom. A caller with no view in mind may pass `x0()` and get the old behaviour.
   ///
   /// Level 0 emits the raw samples of [begin, end). Level >= 1 emits one
   /// geometry element per *complete* level bucket wholly inside the retained
@@ -85,7 +93,7 @@ class Series {
   /// level_for() picks), and never reaches outside the retained window -- nor
   /// outside [begin, end). Pairs are appended to out_xy, which is never
   /// cleared.
-  virtual void gather(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
+  virtual void gather(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
                       GatherMode mode = GatherMode::Envelope) const = 0;
 
   /// Absolute x of sample `i` (`x0() + i*dx()` on a uniform-grid series).
@@ -121,7 +129,7 @@ class Series {
 
   /// The shared body of the concrete gather() overrides: raw samples at level 0,
   /// bucket geometry above it (per `mode`).
-  void gather_impl(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
+  void gather_impl(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
                    GatherMode mode) const;
 
  private:
@@ -143,7 +151,7 @@ class Series {
   /// 0 is entirely this, and a level >= 1 gather uses it for the two partial edge
   /// buckets a pyramid of complete buckets cannot answer for. Precondition:
   /// begin < end, end <= size().
-  void gather_raw(u64 begin, u64 end, std::vector<f32>& out_xy) const;
+  void gather_raw(u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy) const;
 };
 
 }  // namespace plot::data

include/plot/data/stream_series.hpp

@@ -92,7 +92,7 @@ class StreamSeries final : public Series {
   Range x_bounds() const override;
   Range y_bounds() const override;
   const MinMaxPyramid& pyramid() const override;
-  void gather(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
+  void gather(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
               GatherMode mode = GatherMode::Envelope) const override;
   /// Always true: this series' `x_bounds()` is the retained ring window, which
   /// slides with every `acquire()` -- the property `XFollow::Live` follows.

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.