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.

adapters/imgui/imgui_plot.cpp

@@ -201,6 +201,11 @@ void PlotWidget::forward_input(bool hovered) {
     e.down = any_down;
     e.pressed = pressed;
     e.released = released;
+    // ImGui's own clock: monotonic seconds since the context was created. The
+    // double-click detector differences two presses' stamps, so only the unit
+    // matters -- and ImGui's backend-driven time keeps running while the pointer
+    // is off-window, which is exactly what a wall clock should do.
+    e.time_s = ImGui::GetTime();
     controller_->on_pointer(e);
   };
 
@@ -231,6 +236,15 @@ void PlotWidget::forward_input(bool hovered) {
   // Keyboard gestures, on the same hover gate as the pointer: a key pressed
   // while the plot is under the pointer is aimed at the plot. `+` on most
   // layouts is shift+`=`, and either spelling zooms in, as in the GLFW adapter.
+  //
+  // Every mapped key is asked with repeat = true. This is the adapter's whole key
+  // -repeat policy, and the odd one out among the hosts: GLFW, Qt and GTK each
+  // deliver the OS's auto-repeat as extra press events, while ImGui edge-detects
+  // presses once and only re-fires `IsKeyPressed` while the key is *held* if it is
+  // asked to. Holding `+` or an arrow on a plot is a zoom/pan the user expects to
+  // keep going (docs/embedding.md's shared policy: a held key repeats in every
+  // adapter), so the repeat is requested here. Escape repeats harmlessly --
+  // cancelling an already-cancelled drag is a no-op.
   if (hovered && !io.WantTextInput) {
     const auto key = [&](int code) {
       input::KeyEvent e;
@@ -239,15 +253,15 @@ void PlotWidget::forward_input(bool hovered) {
       e.down = true;  // ImGui already edge-detected the press
       controller_->on_key(e);
     };
-    if (ImGui::IsKeyPressed(ImGuiKey_Equal, false) ||
-        ImGui::IsKeyPressed(ImGuiKey_KeypadAdd, false)) {
-      key(input::kKeyZoomIn);
-    }
-    if (ImGui::IsKeyPressed(ImGuiKey_Minus, false) ||
-        ImGui::IsKeyPressed(ImGuiKey_KeypadSubtract, false)) {
-      key(input::kKeyZoomOut);
-    }
-    if (ImGui::IsKeyPressed(ImGuiKey_F, false)) key(input::kKeyFit);
+    const auto pressed = [](ImGuiKey k) { return ImGui::IsKeyPressed(k, true); };
+    if (pressed(ImGuiKey_Equal) || pressed(ImGuiKey_KeypadAdd)) key(input::kKeyZoomIn);
+    if (pressed(ImGuiKey_Minus) || pressed(ImGuiKey_KeypadSubtract)) key(input::kKeyZoomOut);
+    if (pressed(ImGuiKey_F)) key(input::kKeyFit);
+    if (pressed(ImGuiKey_LeftArrow)) key(input::kKeyLeft);
+    if (pressed(ImGuiKey_RightArrow)) key(input::kKeyRight);
+    if (pressed(ImGuiKey_UpArrow)) key(input::kKeyUp);
+    if (pressed(ImGuiKey_DownArrow)) key(input::kKeyDown);
+    if (pressed(ImGuiKey_Escape)) key(input::kKeyEscape);
   }
 
   prev_down_[0] = down[0];