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/render/vertex_array.hpp

@@ -54,20 +54,21 @@ class LineVertexArray {
   gl::VertexArray vao_;
   u32 instances_ = 0;
   /// Instance-buffer generation the attribute pointers currently point into.
-  u32 epoch_ = 0;
+  u64 epoch_ = 0;
 };
 
 /// Vertex array plus instance buffer of the text pipeline: `a_pos` at location 0
 /// (the cell's top-left corner in pixels), the glyph's atlas rect `a_uv` at
-/// location 1 and its rotation basis `a_axis` at location 2, two floats each,
-/// every one advancing once per instance. The layout is the one `TextInstance`
+/// location 1, its rotation basis `a_axis` at location 2 and the run's tint
+/// `a_colour` at location 3 (`vec2`, `vec2`, `vec2`, `vec4` floats), each
+/// advancing once per instance. The layout is the one `TextInstance`
 /// declares and shaders.hpp consumes; a caller never writes a location, stride or
 /// offset, it stages a `TextBatch` and uploads its `data()`.
 class TextVertexArray {
  public:
   bool create(Status* st = nullptr, UploadMode mode = UploadMode::Orphan) noexcept;
 
-  /// Uploads `glyphs` instances from interleaved `x,y,u,v,ax,ay` floats.
+  /// Uploads `glyphs` instances from interleaved `x,y,u,v,ax,ay,r,g,b,a` floats.
   void upload(const f32* interleaved, u32 glyphs) noexcept;
 
   void bind() const noexcept { vao_.bind(); }
@@ -84,7 +85,7 @@ class TextVertexArray {
   TextBuffer buffer_;
   gl::VertexArray vao_;
   u32 instances_ = 0;
-  u32 epoch_ = 0;
+  u64 epoch_ = 0;
 };
 
 }  // namespace plot::render