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.

tests/test_gl_context.cpp

@@ -494,13 +494,16 @@ PLOT_TEST(render_programs_are_inert_until_created) {
   line.set_offset({0.0f, 0.0f});
   line.set_viewport({256.0f, 256.0f});
   line.set_width(2.0f);
+  // The AA fringe is a uniform now, not a hardcoded pixel: 1.0 is the fringe the
+  // renderer's own flush sets by default, and the same value the pixel bounds
+  // below were derived from.
+  line.set_aa(1.0f);
   line.set_color(plot::Color::hex(0xFFFFFF));
   text.bind();
   text.set_viewport({256.0f, 256.0f});
   text.set_px_range({8.0f, 8.0f});
   text.set_glyph_uv({1.0f / 16.0f, 1.0f / 7.0f});
   text.set_texture_size({128.0f, 56.0f});
-  text.set_color(plot::Color::hex(0xFFFFFF));
   PLOT_CHECK(plot::gl::glGetError() == plot::gl::GL_NO_ERROR);
   ctx.destroy();
 }
@@ -541,6 +544,10 @@ PLOT_TEST(line_pipeline_draws_an_aa_strip) {
   program.set_offset({64.0f, 100.0f});
   program.set_viewport({static_cast<f32>(ctx.width), static_cast<f32>(ctx.height)});
   program.set_width(3.0f);
+  // The AA fringe is a uniform now, not a hardcoded pixel: 1.0 is the fringe the
+  // renderer's own flush sets by default, and the value the pixel bounds below
+  // were derived from.
+  program.set_aa(1.0f);
   program.set_color(plot::Color::hex(0xFF00FF));
   lines.bind();
   plot::gl::glDrawArraysInstanced(plot::gl::GL_TRIANGLE_STRIP, 0, 4, 3);
@@ -619,7 +626,7 @@ PLOT_TEST(text_pipeline_draws_a_glyph) {
   // an identity.
   batch.add_text("A", {64.0f, 100.0f}, 1.0f, font);
   PLOT_CHECK(batch.glyph_count() == 1);
-  PLOT_CHECK(batch.data().size() == 6);
+  PLOT_CHECK(batch.data().size() == 10);
   glyphs.upload(batch.data().data(), batch.glyph_count());
 
   plot::gl::glViewport(0, 0, static_cast<plot::gl::GLsizei>(ctx.width),
@@ -640,7 +647,6 @@ PLOT_TEST(text_pipeline_draws_a_glyph) {
   // White, so the blended pixel value *is* the coverage byte: the shader emits
   // `vec4(rgb, a * coverage)`, and a white tint over the black clear lands at
   // `1.0 * coverage` in every channel.
-  program.set_color(plot::Color::hex(0xFFFFFF));
   atlas.bind();
   glyphs.bind();
   plot::gl::glDrawArraysInstanced(plot::gl::GL_TRIANGLE_STRIP, 0, 4, 1);