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/coverage.md

@@ -43,35 +43,36 @@ when one is missing.
 <!-- BEGIN GENERATED: tools/coverage.sh -->
 
 ```
-coverage_floor: ok -- 23 files under src/, aggregate 95.2% lines (3302/3469), every file at or above 80%, aggregate target 90%
+coverage_floor: ok -- 24 files under src/, aggregate 93.9% lines (3730/3971), every file at or above 80%, aggregate target 90%
 ```
 
 | File | Lines | Line rate | Functions |
 | --- | ---: | ---: | ---: |
-| `src/data/array_series.cpp` | 43/43 | 100.0% | 100.0% |
-| `src/data/minmax_pyramid.cpp` | 163/163 | 100.0% | 100.0% |
-| `src/data/series.cpp` | 105/107 | 98.1% | 100.0% |
-| `src/data/stream_series.cpp` | 39/39 | 100.0% | 100.0% |
-| `src/gl/handle.cpp` | 256/256 | 100.0% | 100.0% |
+| `src/data/array_series.cpp` | 56/56 | 100.0% | 100.0% |
+| `src/data/minmax_pyramid.cpp` | 170/171 | 99.4% | 96.4% |
+| `src/data/series.cpp` | 161/164 | 98.2% | 100.0% |
+| `src/data/stream_series.cpp` | 45/45 | 100.0% | 100.0% |
+| `src/data/xy_series.cpp` | 102/120 | 85.0% | 100.0% |
+| `src/gl/handle.cpp` | 259/259 | 100.0% | 100.0% |
 | `src/gl/loader.cpp` | 81/81 | 100.0% | 100.0% |
-| `src/input/controller.cpp` | 329/343 | 95.9% | 100.0% |
+| `src/input/controller.cpp` | 430/465 | 92.5% | 97.4% |
 | `src/math/transform.cpp` | 28/28 | 100.0% | 100.0% |
 | `src/plot/axis.cpp` | 12/12 | 100.0% | 100.0% |
-| `src/plot/figure.cpp` | 218/223 | 97.8% | 95.5% |
+| `src/plot/figure.cpp` | 229/234 | 97.9% | 95.8% |
 | `src/plot/grid.cpp` | 15/16 | 93.8% | 100.0% |
 | `src/plot/measure.cpp` | 21/21 | 100.0% | 100.0% |
-| `src/plot/plot.cpp` | 743/763 | 97.4% | 98.9% |
-| `src/plot/ticks.cpp` | 93/99 | 93.9% | 100.0% |
+| `src/plot/plot.cpp` | 767/793 | 96.7% | 98.9% |
+| `src/plot/ticks.cpp` | 105/111 | 94.6% | 100.0% |
 | `src/render/buffer.cpp` | 88/98 | 89.8% | 100.0% |
 | `src/render/embedded_mono_font.cpp` | 45/45 | 100.0% | 100.0% |
-| `src/render/primitives.cpp` | 71/75 | 94.7% | 100.0% |
+| `src/render/primitives.cpp` | 75/79 | 94.9% | 100.0% |
 | `src/render/program.cpp` | 26/27 | 96.3% | 100.0% |
-| `src/render/renderer.cpp` | 530/609 | 87.0% | 93.7% |
+| `src/render/renderer.cpp` | 616/722 | 85.3% | 93.9% |
 | `src/render/text.cpp` | 27/30 | 90.0% | 100.0% |
 | `src/render/text_atlas.cpp` | 40/41 | 97.6% | 100.0% |
-| `src/render/vertex_array.cpp` | 59/59 | 100.0% | 100.0% |
+| `src/render/vertex_array.cpp` | 62/62 | 100.0% | 100.0% |
 | `src/text/true_type_font.cpp` | 270/291 | 92.8% | 96.8% |
-| **src/ aggregate** | **3302/3469** | **95.2%** | **98.3%** |
+| **src/ aggregate** | **3730/3971** | **93.9%** | **98.0%** |
 
 <!-- END GENERATED: tools/coverage.sh -->