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.

include/plot/gl/api.hpp

@@ -58,8 +58,14 @@ enum : GLenum {
 
   // Blending.
   GL_BLEND = 0x0BE2,
+  GL_ZERO = 0x0000,
+  GL_ONE = 0x0001,
   GL_SRC_ALPHA = 0x0302,
   GL_ONE_MINUS_SRC_ALPHA = 0x0303,
+  GL_BLEND_SRC_RGB = 0x80C9 /* 1.4 */,
+  GL_BLEND_DST_RGB = 0x80C8 /* 1.4 */,
+  GL_BLEND_SRC_ALPHA = 0x80CB /* 1.4 */,
+  GL_BLEND_DST_ALPHA = 0x80CA /* 1.4 */,
 
   // Pixel store state.
   GL_UNPACK_ALIGNMENT = 0x0CF5,
@@ -127,7 +133,9 @@ enum : GLenum {
   X(void, glClearColor, (float, float, float, float))                                 \
   X(void, glClear, (GLbitfield))                                                      \
   X(void, glEnable, (GLenum))                                                         \
+  X(GLboolean, glIsEnabled, (GLenum))                                                 \
   X(void, glBlendFunc, (GLenum, GLenum))                                              \
+  X(void, glBlendFuncSeparate, (GLenum, GLenum, GLenum, GLenum)) /* 1.4 */            \
   X(void, glPixelStorei, (GLenum, GLint))                                             \
   X(void, glDisable, (GLenum))                                                        \
   X(void, glScissor, (GLint, GLint, GLsizei, GLsizei))                                \

include/plot/gl/handle.hpp

@@ -36,7 +36,10 @@ class Handle {
   /// already used, so state that captured the old object -- the attribute
   /// pointers of a vertex array above all -- must compare this counter rather
   /// than the name (see `render::detail::InstanceStore::epoch()`).
-  u32 generation() const noexcept { return generation_; }
+  ///
+  /// 64-bit on purpose: `epoch()` sums this with a slot counter, and a long-lived
+  /// streaming frame loop must not be able to wrap a u32 sum into a stale value.
+  u64 generation() const noexcept { return generation_; }
 
   /// Deletes the GL object, if any, and empties the handle.
   void reset() noexcept {
@@ -73,7 +76,7 @@ class Handle {
   GLuint id_ = 0;
 
  private:
-  u32 generation_ = 0;
+  u64 generation_ = 0;
 };
 
 /// Vertex buffer object. All fallible operations report through `Status`;
@@ -172,6 +175,9 @@ class Texture final : public Handle<Texture> {
   bool create_r8(u32 width, u32 height, Status* st = nullptr) noexcept;
 
   /// Replaces the whole image with `width * height * 4` bytes of RGBA data.
+  /// Sets `GL_UNPACK_ALIGNMENT` to 4 for the upload and restores the caller's
+  /// value afterwards, so a host that shares the context never inherits this
+  /// call's pixel-store state.
   void upload_rgba(u32 width, u32 height, const void* pixels) const noexcept;
 
   /// Replaces the whole image with `width * height` bytes of single-channel