v0.20.0 — arrows, linewidth, hex binning in data units, interactive aesthetics

Minor bump: new public API (arrows, linewidth, hex binning in data units,
interactive aesthetics). LineStyle gains a lineend field, and
GeomSegment/GeomCurve/GeomPath/GeomLine gain arrow/lineend fields,
so struct literals without ..Default::default() need them added.
StatBinHex's fields and output columns are not back-compat with 0.19.0's.
Default output is otherwise unchanged, except hexbin cell geometry and curve
rendering (a native quadratic Bézier <path> instead of a flattened
polyline).

Added

• LineEnd { Butt, Round, Square } and LineStyle::lineend; DrawBackend
  gains a default draw_quadratic(p0, ctrl, p1, style), overridden by the
  SVG backend to emit a native <path d="M.. Q..">.
• src/geom/arrow.rs: Arrow (grid's arrow(): angle, length, ends,
  kind, plus the position/length_per_linewidth extensions) via the
  arrow() builder. GeomSegment/GeomCurve/GeomPath/GeomLine gain
  arrow/lineend fields and .arrow() .lineend() .linewidth() builders
  (curve also .curvature() .ncp()); segment and curve honour per-row
  mapped linetype and alpha (previously ignored).
• Aesthetic::Linewidth ("linewidth") with Aes::linewidth() /
  Aes::after_stat_linewidth(); src/scale/linewidth.rs's
  ScaleLinewidthContinuous (range, limits/squish, transform, breaks,
  labels, locale); GGPlot::scale_linewidth(); a legend key drawn as a
  horizontal line of the mapped width. Segment/curve read it per row;
  path/line per point, drawing segment-by-segment when it varies within a
  group.
• StatBinHex rewritten onto a fixed pointy-top hex lattice
  (src/stat/binhex::HexLattice; cube-rounded point assignment, locate(),
  deterministic binwidth/origin rules) instead of floor-binned
  staggered rectangles anchored at the data minimum. New output columns:
  count, density, ncount, ndensity, width, height, n, q, r (fill
  stays as count, back-compat). GeomHex draws each hexagon's vertices
  in data space (from width/height, falling back to the data's
  resolution) and projects each individually — correct under any coord,
  including coord_sf.
• src/scale/identity.rs: ScaleColorIdentity reads #RRGGBB/#RGB hex
  strings directly, with no legend; GGPlot::scale_colour_identity() /
  scale_color_identity() / scale_fill_identity().
• Interactive aesthetics: Aesthetic::Tooltip (Aes::tooltip(col), a
  custom <title>) and Aesthetic::DataAttr (Aes::data_attr(name, col),
  a custom data-<name> attribute; name must match [a-z0-9][a-z0-9-]*).
  DrawBackend::set_mark_attrs/set_mark_part (the latter also used by
  arrowheads: data-part="arrow" aria-hidden="true", no <title>). A user
  attr replaces a built-in of the same name rather than duplicating it.
• CoordSf::xlim/ylim (zoom, no filtering) and expand(bool), via a new
  default Scale::set_expand(mult, add); GGPlot::coord_sf_with(CoordSf)
  and svg_root_data(bool) (default true; off omits data-plot/
  data-domain/etc from the root <svg>) — for a consumer overlaying a
  rendered fragment on its own basemap.

Changed

• Hexbin cell geometry: StatBinHex no longer floor-bins staggered
  rectangles anchored at the data minimum, so cell positions differ from
  0.19.0 (deterministic, but not reproducible against old output). See
  "Added" above.
• geom_curve draws a native quadratic Bézier <path d="M.. Q..">
  instead of a flattened <polyline>; ncp is now ignored by SVG
  (documented) since a single control point fully determines the curve.

Breaking

• LineStyle gains lineend: LineEnd; GeomSegment, GeomCurve,
  GeomPath, GeomLine gain arrow: Option<Arrow> and
  lineend: LineEnd; StatBinHex's fields and output columns are not
  back-compat with 0.19.0. Struct literals need ..Default::default().
• Aesthetic gains Linewidth, Tooltip and DataAttr(String)
  variants — an exhaustive match on Aesthetic needs a catch-all or the
  new arms.
• Aes gains a private warnings field (build warnings from
  Aes::data_attr, e.g. an invalid or reserved name). A struct literal
  that names every field explicitly — Aes { mappings, after_scale },
  without ..Default::default() — no longer compiles (E0451, "field
  warnings of struct Aes is private"); add ..Default::default().
• ggplot_rs::aes::{resolve_mappings, apply_after_stat} (public
  re-exports of aes::mapping's functions) gain a &mut Vec<String>
  warnings parameter: resolve_mappings(data, mapping) →
  resolve_mappings(data, mapping, warnings), same for
  apply_after_stat. Needed so a data_attr/tooltip mapping that
  turns out invalid or reserved at this stage (not just at
  Aes::data_attr build time — e.g. one constructed by pushing
  directly into Aes.mappings) can still report a warning instead of
  resolving silently.