feat(plot)!: harden pipeline, XY series, dash/markers, touch, packaging

Breaking: Series::gather emits x relative to x0() and takes a GatherMode;
consumers reconstruct absolute x in f64 before mapping (series.hpp documents
the contract).

Bugs, each pinned by a fail-before regression test:
- render: f32 staging quantised epoch-time x into 128 px columns; every
  data-space draw now stages to_space(x) - to_space(origin) with the origin
  folded into the offset uniform in f64 (epoch/zero-origin twins rasterise
  identically)
- ticks: major test was ULP-fragile and blanked epoch axes; majors are an
  integer-index test and are never truncated by the minor cap
- gather: LOD envelope emitted buckets straddling [begin,end); ceil/floor
  bucket edges + raw lead/trail spans
- live follow: log x axes held a linear span and zoomed every frame; span and
  offset now live in axis space (plot + both figure link-group paths)
- input: a zero-notch wheel disarmed auto_scale on every axis; dy==0 pans x,
  factor == 1 / inf refused in wheel and drag zoom
- render: the single text flush painted legend names over the crosshair
  readout panel; two batches, under flushed before the panel
- data: NaN dropped from min/max unless it seeded the fold; non-finite
  samples are now excluded from every aggregate and from cached bounds

Robustness: viewport normalisation (no clamp UB), non-finite range refusal,
pyramid level cap at 57 (no shift/div UB through public gather), rounded ring
capacity documented + pre-sized, GL blend func and UNPACK_ALIGNMENT
save/restore, Renderer::release_gl() teardown, u64 buffer epochs, annotation
captions clamped, button-aware GLFW row capture, cached figure row rects (no
per-event layout), ruler-pair measurement refused, span-nonfinite guards in
axis_ends.

Features: XY series (sorted x, binary-search neighbours/snap, LOD over sample
index); StreamSeries::freeze(); ArraySeries::push(); per-trace dash and
markers (scatter at width 0) with auto or literal stride; GatherMode
Envelope/Nearest/Mean via bucket_sum/sumsq; Style::line_aa_px; per-glyph text
colour (readout values in trace colour); wheel-dx x pan; arrow-key pan;
Escape cancels a drag and restores captured ranges; modifier-masked box zoom
(Shift both, Ctrl x, Alt y); double-click reset via PointerEvent::time_s;
ImGui key repeat unified; pinch gestures on Qt and GTK; pixelDelta wheel path
and DevicePixelRatioChange handling; GNUInstallDirs, SOVERSION,
SameMinorVersion, generated plot/version.hpp, relocatable plot.pc, licence
install, FILES_MATCHING, GitHub Actions CI (debug/coverage/asan).

Suite 283 -> 350 cases, 0 failed; coverage 93.9% (3730/3971) over 24 files,
floor 80/90 held; ASAN preset suite green; all four demos run headless (the
GTK scripted-resize check needs a window manager and fails identically on the
parent commit); readme_main.cpp committed and compiles with the README
command; pkg-config and find_package consumption verified from a relocated
install prefix.

include/plot/data/array_series.hpp

@@ -22,6 +22,10 @@ class ArraySeries final : public Series {
   void reset(std::vector<f32> y, f64 x0, f64 dx);
   /// Rebuilds the pyramid and cached bounds from y_ as it stands now.
   void commit();
+  /// Appends one sample incrementally: the pyramid folds it (`append`, amortised
+  /// O(1)) and the cached bounds extend in O(1), NaN-aware, instead of commit()
+  /// rebuilding the whole store. The wholesale path stays `commit()`.
+  void push(f32 y);
 
   u64 size() const override;
   f64 x0() const override;
@@ -29,7 +33,8 @@ 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) const override;
+  void gather(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
+              GatherMode mode = GatherMode::Envelope) const override;
 
  protected:
   void copy_raw(u64 begin, u64 end, f32* out) const override;
@@ -41,6 +46,11 @@ class ArraySeries final : public Series {
   MinMaxPyramid pyramid_;
   Range x_bounds_{};
   Range y_bounds_{};
+  /// False until a finite sample has been seen: `y_bounds_` is then a
+  /// placeholder (zero-span), not an aggregate, and the next finite sample
+  /// seeds it. This is what makes push()'s incremental extension and commit()'s
+  /// rebuild agree on a window that holds non-finite samples.
+  bool y_has_finite_ = false;
 };
 
 }  // namespace plot::data