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.

src/render/vertex_array.cpp

@@ -72,15 +72,18 @@ bool TextVertexArray::create(Status* st, UploadMode mode) noexcept {
   }
   vao_.bind();
   buffer_.bind();
-  // Three attributes now: the cell's corner, its atlas rect and the basis that
-  // rotates it (see TextInstance). All of them advance once per instance, like
-  // the line pipeline's, because the corners come from gl_VertexID.
+  // Four attributes now: the cell's corner, its atlas rect, the basis that
+  // rotates it and the run's tint (see TextInstance). All of them advance once
+  // per instance, like the line pipeline's, because the corners come from
+  // gl_VertexID.
   vao_.attrib(0u, 2, sizeof(TextInstance), offsetof(TextInstance, x));
   vao_.attrib(1u, 2, sizeof(TextInstance), offsetof(TextInstance, u));
   vao_.attrib(2u, 2, sizeof(TextInstance), offsetof(TextInstance, ax));
+  vao_.attrib(3u, 4, sizeof(TextInstance), offsetof(TextInstance, r));
   gl::glVertexAttribDivisor(0u, kPerInstance);
   gl::glVertexAttribDivisor(1u, kPerInstance);
   gl::glVertexAttribDivisor(2u, kPerInstance);
+  gl::glVertexAttribDivisor(3u, kPerInstance);
   gl::glBindVertexArray(0);
   epoch_ = buffer_.epoch();
   return true;
@@ -99,6 +102,7 @@ void TextVertexArray::upload(const f32* interleaved, u32 glyphs) noexcept {
     vao_.attrib(0u, 2, sizeof(TextInstance), base + offsetof(TextInstance, x));
     vao_.attrib(1u, 2, sizeof(TextInstance), base + offsetof(TextInstance, u));
     vao_.attrib(2u, 2, sizeof(TextInstance), base + offsetof(TextInstance, ax));
+    vao_.attrib(3u, 4, sizeof(TextInstance), base + offsetof(TextInstance, r));
     epoch_ = buffer_.epoch();
   }
   instances_ = glyphs;