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.

cmake/plot_options.cmake

@@ -16,6 +16,30 @@ option(PLOT_BUILD_GLFW "Build the GLFW adapter and demo" OFF)
 option(PLOT_BUILD_IMGUI "Build the Dear ImGui adapter and demo" OFF)
 option(PLOT_BUILD_QT6 "Build the Qt6 adapter and demo" OFF)
 option(PLOT_BUILD_GTK4 "Build the GTK4 adapter and demo" OFF)
+# 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. The top-level CMakeLists.txt adds
+# the target itself; this file only decides whether it exists.
+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)"
+  ${PLOT_VENDORED_STB_TRUETYPE})
+if(PLOT_BUILD_TTF_FONT AND NOT PLOT_VENDORED_STB_TRUETYPE)
+  message(FATAL_ERROR
+    "plot: PLOT_BUILD_TTF_FONT=ON needs third_party/stb_truetype.h, which this tree does "
+    "not have. Restore it -- include/plot/text/true_type_font.hpp records the version and "
+    "the source URL it came from -- or configure with -DPLOT_BUILD_TTF_FONT=OFF.")
+endif()
 option(PLOT_INSTALL "Generate install and export rules (find_package(plot))" ON)
 set(PLOT_SANITIZE "" CACHE STRING
     "Sanitizers to build with, as the list a -fsanitize= flag takes (e.g. address); empty for none")
@@ -27,9 +51,22 @@ set(PLOT_SANITIZE "" CACHE STRING
 # presets leave it off, so their target lists, configure time and build product
 # are exactly what they were before this option existed.
 option(PLOT_COVERAGE "Add the `coverage` target (needs a --coverage build tree)" OFF)
-if(PLOT_COVERAGE AND NOT CMAKE_CXX_FLAGS MATCHES "--coverage")
+# The instrumented tree is one whose *effective* flags carry --coverage, and the
+# base CMAKE_CXX_FLAGS is only part of those: the active configuration's own
+# profile (CMAKE_CXX_FLAGS_<CONFIG>) is applied on top, and --coverage is
+# equally at home in either. The linux-coverage preset sets the base; a user who
+# sets it on the Debug profile instead gets exactly the same instrumented tree,
+# which the old base-only string match rejected. Matching the two together is
+# still just an option check on the configuration profile -- the guard does not
+# try to read the compiler's mind, only the flags this build will pass it.
+string(TOUPPER "${CMAKE_BUILD_TYPE}" plot_coverage_config)
+set(plot_coverage_flags
+  "${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_${plot_coverage_config}}")
+if(PLOT_COVERAGE AND NOT plot_coverage_flags MATCHES "--coverage")
   message(FATAL_ERROR
-    "plot: PLOT_COVERAGE=ON needs a --coverage build (CMAKE_CXX_FLAGS is \"${CMAKE_CXX_FLAGS}\"). "
+    "plot: PLOT_COVERAGE=ON needs a --coverage build (CMAKE_CXX_FLAGS is "
+    "\"${CMAKE_CXX_FLAGS}\" and CMAKE_CXX_FLAGS_${plot_coverage_config} is "
+    "\"${CMAKE_CXX_FLAGS_${plot_coverage_config}}\"). "
     "Configure with --preset linux-coverage, or drop -DPLOT_COVERAGE=ON.")
 endif()
 if(PLOT_COVERAGE AND NOT PLOT_BUILD_TESTS)