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/building-and-testing.md

@@ -41,9 +41,27 @@ ctest --test-dir build/debug --output-on-failure
 
 ctest registers four tests: `plot_tests` (the test binary), the two header/umbrella build
 checks (`plot_headers_self_contained`, `plot_umbrella_covers_public_headers`) and
-`plot_umbrella_check`. The `plot_tests` binary itself runs **283 cases** in the `linux-debug`
-preset; its harness prints one line per case and a final `283 cases, 0 failed` summary line.
-The case counts per file are the `PLOT_TEST(...)` declarations under `tests/`.
+`plot_umbrella_check`. The `plot_tests` binary itself runs **350 cases** in the `linux-debug`
+preset; its harness prints one line per case and a final `350 cases, 0 failed` summary line.
+The case counts per file are the `PLOT_TEST(...)` declarations under `tests/` — recount them
+(`grep -c '^PLOT_TEST(' tests/test_*.cpp`) after adding or removing cases.
+
+## Install layout
+
+`cmake --install` follows `GNUInstallDirs`, so every path honours the standard cache
+variables (`CMAKE_INSTALL_LIBDIR`, `CMAKE_INSTALL_INCLUDEDIR`, …) and their distribution
+defaults — plain `lib`, `include` and `share` unless a packager overrides them:
+
+| prefix-relative path | what |
+|---|---|
+| `lib/` | the `plot_core` archive (and `libplot_core.so` with its version and SONAME symlinks when built with `BUILD_SHARED_LIBS=ON`) |
+| `include/plot/` | every public header, including the generated `plot/version.hpp` (`plot::version::major/minor/patch/string` and the `PLOT_VERSION_*` macros) |
+| `lib/cmake/plot/` | the CMake package: `plotConfig.cmake`, `plotConfigVersion.cmake` (Compatibility **SameMinorVersion**, so `find_package(plot 0.1 …)` accepts 0.1.x only), `plotTargets.cmake` |
+| `lib/pkgconfig/plot.pc` | pkg-config metadata for consumers whose build is not CMake |
+| `share/licenses/plot/` | `LICENSE` (MIT) and `licences/OFL-1.1.txt` (the embedded font) |
+
+Adapters that were built and exported install themselves alongside the core (their headers
+under `include/<adapter>/`, their targets in the same package).
 
 ## Headless demo modes
 
@@ -95,15 +113,15 @@ The floors are enforced by `tools/coverage_floor.py`:
 | every file under `src/` | **80%** of its instrumented lines |
 | `src/` as a whole | **90%** of instrumented lines |
 
-Branch coverage is deliberately off. The current measurement (from `build/cov/coverage.info`,
-re-read with `python3 tools/coverage_floor.py build/cov/coverage.info`) is an **aggregate
-95.3% (3284/3447 lines)** over 23 files, every file at or above 80% — the floor is satisfied.
-The method, the full per-file table and the reproducibility notes are in
-[docs/coverage.md](coverage.md); re-run the command for the live numbers.
+Branch coverage is deliberately off. This page carries no measured totals — a number written
+here would be stale the moment `src/` moved, and the report is reproducible: the method, the
+per-file table and the floor verdict live in [docs/coverage.md](coverage.md), which
+`tools/coverage.sh` rewrites from the very run that measured them. Re-run the command for the
+live numbers.
 
-**One thing to know about the coverage preset:** it builds **272 of the 283 cases**. The 11
-that are missing are the adapter cases that need an adapter target to exist — 8 in
-`test_imgui_widget.cpp` and 3 in `test_glfw_host.cpp` — and the coverage preset turns the
-adapters off. The toolkit-free adapter seams still run there: `test_qt_pointer.cpp` (2 cases)
-and `test_gtk_input.cpp` (8 cases) compile without Qt or GTK. So the coverage number measures
+**One thing to know about the coverage preset:** it builds **333 of the 350 cases**. The 17
+that are missing are the adapter cases that need an adapter target to exist — 11 in
+`test_imgui_widget.cpp` and 6 in `test_glfw_host.cpp` — and the coverage preset turns the
+adapters off. The toolkit-free adapter seams still run there: `test_qt_pointer.cpp` (5 cases)
+and `test_gtk_input.cpp` (11 cases) compile without Qt or GTK. So the coverage number measures
 the library plus its unit suite, not the adapter bodies (which the headless demo runs cover).