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.

CMakeLists.txt

@@ -1,8 +1,19 @@
 cmake_minimum_required(VERSION 3.24)
-project(plot VERSION 0.1.0 LANGUAGES CXX)
+project(plot VERSION 0.1.0
+  DESCRIPTION "An embeddable OpenGL time-series plotting library for real-time, high-sample-rate data"
+  LANGUAGES CXX)
 
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
 include(plot_options)
+include(GNUInstallDirs)
+
+# The version header: generated into the build tree at configure time, from the
+# template beside it. It is a public header -- the umbrella names it, the
+# self-containment and umbrella checks in tests/ hold it to the same rules, and
+# the install rules place it under include/plot/ with the rest of the surface --
+# so a consumer can ask what it compiled against (see include/plot/version.hpp.in).
+configure_file(include/plot/version.hpp.in
+  "${CMAKE_CURRENT_BINARY_DIR}/include/plot/version.hpp" @ONLY)
 
 add_library(plot_core
   src/core/status.cpp
@@ -11,6 +22,7 @@ add_library(plot_core
   src/data/series.cpp
   src/data/array_series.cpp
   src/data/stream_series.cpp
+  src/data/xy_series.cpp
   src/gl/loader.cpp
   src/gl/handle.cpp
   src/render/shaders.cpp
@@ -36,36 +48,32 @@ add_library(plot::core ALIAS plot_core)
 # and every consumer, and this project's own comments -- says plot::core.
 set_target_properties(plot_core PROPERTIES EXPORT_NAME core)
 
+# The shared-library case names its file versions: the dynamic loader follows
+# the SONAME (libplot_core.so.<SOVERSION>, the major component), so an
+# ABI-breaking release moves it by bumping project()'s major. The static
+# archive -- the default, and the only shape the presets build -- carries no
+# such name, and setting the properties on it would be noise.
+if(BUILD_SHARED_LIBS)
+  set_target_properties(plot_core PROPERTIES
+    VERSION ${PROJECT_VERSION}
+    SOVERSION ${PROJECT_VERSION_MAJOR})
+endif()
+
 target_include_directories(plot_core PUBLIC
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
+  # The generated version header lives here, not under the source root (see
+  # configure_file above); everything that compiles against plot::core reaches
+  # it through this interface, including the header checks in tests/.
+  $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/include>
   $<INSTALL_INTERFACE:include>)
 target_compile_features(plot_core PUBLIC cxx_std_20)
 target_link_libraries(plot_core PRIVATE ${CMAKE_DL_LIBS})
 plot_apply_warnings(plot_core)
 
-# The optional runtime-font backend: plot::text::TrueTypeFont, which bakes a real
-# TrueType/OpenType file into a fixed-cell atlas at run time (see
-# include/plot/text/true_type_font.hpp). It is the one target that compiles the
-# vendored third_party/stb_truetype.h, so it is opt-in and separable -- the core
-# library, its public headers and its tests build without it -- and its default
-# follows the vendored header's presence: a tree that has the file wants the
-# feature, and one that does not cannot have it. Asking for it explicitly without
-# the header is an error rather than a silent downgrade, because the option is
-# then the only thing that would have said so.
-if(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/third_party/stb_truetype.h")
-  set(PLOT_VENDORED_STB_TRUETYPE TRUE)
-else()
-  set(PLOT_VENDORED_STB_TRUETYPE FALSE)
-endif()
-option(PLOT_BUILD_TTF_FONT
-  "Build the optional plot::text::TrueTypeFont backend (vendored stb_truetype)"
-  ${PLOT_VENDORED_STB_TRUETYPE})
-if(PLOT_BUILD_TTF_FONT AND NOT PLOT_VENDORED_STB_TRUETYPE)
-  message(FATAL_ERROR
-    "plot: PLOT_BUILD_TTF_FONT=ON needs third_party/stb_truetype.h, which this tree does "
-    "not have. Restore it -- include/plot/text/true_type_font.hpp records the version and "
-    "the source URL it came from -- or configure with -DPLOT_BUILD_TTF_FONT=OFF.")
-endif()
+# The optional runtime-font backend: plot::text::TrueTypeFont. The option and
+# its guard live in cmake/plot_options.cmake with every other PLOT_* option;
+# what is left here is the one thing an option cannot do for itself: adding the
+# target. See the option's comment there for why it is opt-in and separable.
 if(PLOT_BUILD_TTF_FONT)
   add_subdirectory(src/text)
 endif()
@@ -162,35 +170,70 @@ if(PLOT_INSTALL)
   # asking for it twice puts the same directory in the export twice.
   install(TARGETS plot_core ${PLOT_ADAPTER_EXPORT_TARGETS}
     EXPORT plotTargets
-    ARCHIVE DESTINATION lib
-    LIBRARY DESTINATION lib
-    RUNTIME DESTINATION bin)
-  # The optional backend's header goes in only when that backend was built: an
-  # installed header with no library behind it is a promise the prefix cannot keep,
-  # and which configuration a prefix came from is not something a consumer can see.
+    ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+    RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
+  # The public headers, and the generated version header with them. The
+  # directory rule takes the source tree (FILES_MATCHING, so the version.hpp.in
+  # template -- a .in, not a .hpp -- cannot travel); the generated
+  # include/plot/version.hpp is not under the source root, so it goes in by
+  # name. The optional backend's header goes in only when that backend was
+  # built: an installed header with no library behind it is a promise the
+  # prefix cannot keep, and which configuration a prefix came from is not
+  # something a consumer can see.
   if(PLOT_BUILD_TTF_FONT)
-    install(DIRECTORY include/plot DESTINATION include)
+    install(DIRECTORY include/plot
+      DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
+      FILES_MATCHING PATTERN "*.hpp")
   else()
-    install(DIRECTORY include/plot DESTINATION include
+    install(DIRECTORY include/plot
+      DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
+      FILES_MATCHING PATTERN "*.hpp"
       PATTERN "true_type_font.hpp" EXCLUDE)
   endif()
+  install(FILES "${CMAKE_CURRENT_BINARY_DIR}/include/plot/version.hpp"
+    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/plot)
+
+  # The licences that travel with the installed files: this library's MIT text,
+  # and the OFL 1.1 text for the embedded font the headers bake in, so a
+  # redistributor of the prefix holds both grants without a second checkout.
+  install(FILES LICENSE licences/OFL-1.1.txt
+    DESTINATION ${CMAKE_INSTALL_DATADIR}/licenses/plot)
+
+  # pkg-config metadata, for consumers whose build is not CMake. find_package()
+  # and the package config below remain the primary interface; see plot.pc.in.
+  set(PLOT_PC_LIBDIR "${CMAKE_INSTALL_LIBDIR}")
+  set(PLOT_PC_INCLUDEDIR "${CMAKE_INSTALL_INCLUDEDIR}")
+  set(PLOT_PC_LIB "-lplot_core")
+  set(PLOT_PC_LIBS_PRIVATE "")
+  foreach(plot_pc_lib IN LISTS CMAKE_DL_LIBS)
+    list(APPEND PLOT_PC_LIBS_PRIVATE "-l${plot_pc_lib}")
+  endforeach()
+  string(REPLACE ";" " " PLOT_PC_LIBS_PRIVATE "${PLOT_PC_LIBS_PRIVATE}")
+  configure_file(plot.pc.in "${CMAKE_CURRENT_BINARY_DIR}/plot.pc" @ONLY)
+  install(FILES "${CMAKE_CURRENT_BINARY_DIR}/plot.pc"
+    DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)
 
   install(EXPORT plotTargets
     NAMESPACE plot::
     FILE plotTargets.cmake
-    DESTINATION lib/cmake/plot)
+    DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/plot)
 
   configure_package_config_file(
     cmake/plotConfig.cmake.in
     "${CMAKE_CURRENT_BINARY_DIR}/plotConfig.cmake"
-    INSTALL_DESTINATION lib/cmake/plot)
+    INSTALL_DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/plot)
+  # SameMinorVersion, deliberately: the project is 0.x, where the major
+  # component carries no ABI promise and the minor is what moves when one
+  # breaks, so a consumer asking for 0.1 gets 0.1.x and nothing newer. It goes
+  # up to SameMajorVersion when the version leaves 0.
   write_basic_package_version_file(
     "${CMAKE_CURRENT_BINARY_DIR}/plotConfigVersion.cmake"
     VERSION ${PROJECT_VERSION}
-    COMPATIBILITY SameMajorVersion)
+    COMPATIBILITY SameMinorVersion)
 
   install(FILES
     "${CMAKE_CURRENT_BINARY_DIR}/plotConfig.cmake"
     "${CMAKE_CURRENT_BINARY_DIR}/plotConfigVersion.cmake"
-    DESTINATION lib/cmake/plot)
+    DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/plot)
 endif()