fix: a zoomed trace is drawn on the x its readout quotes

gather() staged x as f32 relative to the series' own x0() -- the start of
the whole retained window -- so the quantisation was a fraction of the
ring rather than of the view: 1.9 us at 20 s of retained data, which at a
10 us view across a 716 px axis is over 100 px between where a sample is
drawn and the x a readout quotes for it (crosshair_point() reads x_at()
in exact f64). It halved on every zoom out, which is what made it read as
"zooming in makes the crosshair select a point beside the data".

The origin is now the caller's (plotter c2c8c59): the renderer passes the
visible range's minimum, so the error is a fixed 2^-24 of what is on
screen -- about 4e-5 px over a 700 px axis -- at any zoom. The parameter
sits before out_xy so every call site is a compile error rather than a
silent change of behaviour.

The renderer no longer reads series.x0() at paint time at all, which
removes two of the three symptoms of the torn window_x0_ read by
construction; the ink probe's now-vacuous kept_fixed_x0 walk goes with
them, replaced by a drawn-vs-quoted pixel figure. neighbours() still
tests the visible range against x_at(0), so the cache stays load-bearing.

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

src/event_series.hpp

@@ -108,7 +108,7 @@ class EventSeries final : public plot::data::Series {
   /// Indices need no rebasing: the pyramid is built from sample 0 and nothing is ever
   /// dropped from it, so `window_base()` is 0 and a window index *is* the absolute one
   /// (unlike the ring, whose window slides).
-  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,
               plot::data::GatherMode mode =
                   plot::data::GatherMode::Envelope) const override {
     if (channel_ >= snap_->values.size()) return;
@@ -118,17 +118,17 @@ class EventSeries final : public plot::data::Series {
     const u64 b = std::min(begin, n);
     const u64 e = std::min(std::max(end, b), n);
     if (b >= e) return;
-    // x **relative to x0()**, which is the library's gather contract: the renderer
-    // adds `x0()` back before mapping to the screen. Emitting the absolute stamp made
-    // it add x0 twice, so the whole capture was drawn one `x0()` -- a full
-    // pre-trigger window -- to the left of the axis it had just been measured
-    // against. The event's own stamps are small (edge-relative, so [-pre, +post]),
-    // which is why that shift looked like a plausible picture: with a 50/50 split the
-    // right half of the plot was simply empty, zooming out showed the trace ending at
-    // the x = 0 marker, and zooming in past the shift showed nothing at all while the
-    // crosshair -- which reads `x_at()`, never `gather()` -- went on reporting the
-    // right values.
-    const f64 base = x0();
+    // x **relative to `x_origin`**, which is the library's gather contract: the
+    // renderer adds that same origin back before mapping to the screen. Emitting the
+    // absolute stamp made it add an origin twice, so the whole capture was drawn one
+    // `x0()` -- a full pre-trigger window -- to the left of the axis it had just been
+    // measured against. The event's own stamps are small (edge-relative, so
+    // [-pre, +post]), which is why that shift looked like a plausible picture: with a
+    // 50/50 split the right half of the plot was simply empty, zooming out showed the
+    // trace ending at the x = 0 marker, and zooming in past the shift showed nothing
+    // at all while the crosshair -- which reads `x_at()`, never `gather()` -- went on
+    // reporting the right values.
+    const f64 base = x_origin;
     const auto rel = [&](u64 i) {
       return static_cast<f32>(stamps[static_cast<std::size_t>(i)] - base);
     };