v0.28.1

Pattern fills (#206): a stripe Pattern as na_pattern on ScaleFillBinned,
drawn in the legend's NA key and by geom_sf / polygon / rect / tile (SVG
<defs><pattern> with per-plot ids; raster stripes; plotters falls back to the
background colour), for hatched no-data regions on choropleths
(DataZooDE/anofox-visualization#420). No API breaks (cargo-semver-checks
against v0.28.0).

Added

• Pattern fills (#206): render::backend::FillPattern (Stripe { fg, bg, size_px, angle_deg }, a thin 30%-duty-cycle hatch by default —
  FillPattern::stripe(fg, bg)), ScaleFillBinned::na_pattern(..) (sits
  alongside the existing na_color, drawn instead of it), and three new
  DrawBackend methods — draw_rect_pattern/draw_polygon_pattern/
  draw_polygon_rings_pattern — drawn by geom_sf/geom_polygon/
  geom_rect/geom_tile for a Value::Na row and by the legend's own
  "no data" key, instead of na_color's flat colour. The native SVG
  backend registers a per-backend-instance-prefixed <defs><pattern>
  (deduped by equality) and fills with its url(#…); the pixel/canvas
  backend rasterises the actual stripes per pixel; plotters has no
  pattern primitive and keeps the trait methods' default — a flat
  FillPattern::bg fill, so a no-data region still reads as muted, just
  without the hatch. Holes still cut out under the even-odd rule exactly
  as a flat fill would. DataZooDE/anofox-visualization#420 — a classed
  choropleth's no-data cells can retire their hand-written SVG <pattern>
  hatch (map/mod.rs) onto this instead of losing it to a flat na colour
  (a visible regression the ::MAP → geom_sf port would otherwise be
  stuck with).