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.

docs/getting-started.md

@@ -51,6 +51,29 @@ directory as its public interface and nothing else. The exported adapters are na
 `plot::imgui` is a build-tree target only, because ImGui is supplied by `FetchContent` and
 there is no ImGui package for an installed config file to point at.
 
+The version you ask for follows the package's own compatibility rule, **SameMinorVersion**:
+`find_package(plot 0.1 REQUIRED)` accepts 0.1.x and refuses 0.2+, because at 0.x the minor
+component is what moves when the surface breaks. What the surface *is* — the version this
+header set was generated from — is in `<plot/version.hpp>`, installed with the other public
+headers: `plot::version::major`, `::minor`, `::patch` and `::string` (plus the
+`PLOT_VERSION_*` macros) for a compile-time answer, no `pkg-config --modversion` round-trip
+needed:
+
+```cpp
+#include <plot/version.hpp>
+
+static_assert(plot::version::major == 0, "plot 0.x only");
+```
+
+A build that is not CMake can use the installed pkg-config file instead:
+
+```sh
+pkg-config --cflags --libs plot   # -I<includedir> -L<libdir> -lplot_core
+```
+
+Both grants travel with the install: `LICENSE` (MIT) and the embedded font's
+`licences/OFL-1.1.txt` (SIL OFL 1.1) are under `share/licenses/plot/` in the prefix.
+
 Every public component is reachable through the umbrella header:
 
 ```cpp
@@ -68,10 +91,11 @@ and draws one trace. It uses the GLFW toolkit for the window and `plot::core` di
 it does not use the shipped `plot::glfw::Host` adapter, so it also shows the pieces the
 adapter hides (window, context, loader, renderer, controller, frame loop).
 
-It was compiled and linked as part of writing this page:
+It was compiled and linked as part of writing this page (`-I build/debug/include` is where
+CMake generates `plot/version.hpp`, which the umbrella pulls in):
 
 ```sh
-g++ -std=c++20 -I include main.cpp build/debug/libplot_core.a \
+g++ -std=c++20 -I include -I build/debug/include main.cpp build/debug/libplot_core.a \
     $(pkg-config --libs glfw3) -ldl -lpthread -o plot_quickstart
 ```