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/CMakeLists.txt

@@ -32,6 +32,7 @@ add_executable(plot_tests
   test_ring_buffer.cpp
   test_minmax_pyramid.cpp
   test_series.cpp
+  test_xy_series.cpp
   test_ticks.cpp
   test_measure.cpp
   test_plot.cpp
@@ -77,8 +78,8 @@ target_include_directories(plot_tests PRIVATE "${CMAKE_SOURCE_DIR}/adapters")
 
 # The optional runtime-font backend brings its own cases, and they exist only when
 # its target does: the header, the class and the archive are all absent otherwise,
-# so there would be nothing to compile them against (see PLOT_BUILD_TTF_FONT in the
-# top-level CMakeLists.txt). Guarded on the target rather than on the option, like
+# so there would be nothing to compile them against (see PLOT_BUILD_TTF_FONT in
+# cmake/plot_options.cmake). Guarded on the target rather than on the option, like
 # the adapter cases above, so a target that exists for any reason gets its cases.
 #
 # Two paths are handed over as definitions because a case needs them and should
@@ -157,6 +158,18 @@ foreach(plot_header IN LISTS plot_public_headers)
   list(APPEND plot_header_check_sources "${plot_header_check_source}")
 endforeach()
 
+# The generated version header is public (the umbrella names it) but is not
+# under the source include root -- the top-level CMakeLists.txt configures it
+# into <build>/include/plot/version.hpp -- so the glob above cannot see it. It
+# gets the same standalone-compile TU by hand, and reaches the header through
+# plot::core's build-tree include interface.
+file(GENERATE
+  OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/header_checks/plot_version.cpp"
+  CONTENT
+  "// Generated by tests/CMakeLists.txt: <plot/version.hpp> must compile on its own.\n#include <plot/version.hpp>\n")
+list(APPEND plot_header_check_sources
+  "${CMAKE_CURRENT_BINARY_DIR}/header_checks/plot_version.cpp")
+
 add_library(plot_header_self_containment OBJECT ${plot_header_check_sources})
 target_link_libraries(plot_header_self_containment PRIVATE plot::core)
 plot_apply_warnings(plot_header_self_containment)
@@ -189,8 +202,12 @@ plot_apply_warnings(plot_umbrella_check)
 
 # Coverage check on the umbrella. Included here so a public header missing from
 # plot.hpp stops the configure; the ctest entry below runs the same script again
-# so the check is also a visible test.
+# so the check is also a visible test. The generated version header is passed in
+# both times: the script must require it of the umbrella, and must look for the
+# file in the build tree where configure_file() put it.
 set(PLOT_INCLUDE_DIR "${CMAKE_SOURCE_DIR}/include")
+set(PLOT_GENERATED_INCLUDE_DIR "${CMAKE_BINARY_DIR}/include")
+set(PLOT_EXTRA_PUBLIC_HEADERS "plot/version.hpp")
 set(PLOT_UMBRELLA "${CMAKE_SOURCE_DIR}/include/plot/plot.hpp")
 include("${CMAKE_SOURCE_DIR}/cmake/plot_umbrella_check.cmake")
 
@@ -199,6 +216,7 @@ add_test(NAME plot_headers_self_contained
 add_test(NAME plot_umbrella_covers_public_headers
   COMMAND ${CMAKE_COMMAND}
           -DPLOT_INCLUDE_DIR=${CMAKE_SOURCE_DIR}/include
-          -DPLOT_UMBRELLA=${CMAKE_SOURCE_DIR}/include/plot/plot.hpp
+          -DPLOT_GENERATED_INCLUDE_DIR=${CMAKE_BINARY_DIR}/include
+          -DPLOT_EXTRA_PUBLIC_HEADERS=plot/version.hpp
           -P "${CMAKE_SOURCE_DIR}/cmake/plot_umbrella_check.cmake")
 add_test(NAME plot_umbrella_check COMMAND plot_umbrella_check)