v0.21.0 — SVG mark contract

Minor bump: the SVG mark contract. Default output is byte-identical except
for four deliberate changes: fill-opacity/stroke-opacity omitted at
1.000, draw_line splitting into separate <polyline>s at non-finite
vertices, every data mark gaining a gg-mark gg-<kind> class, and GeomText
honouring a theme/geom font family it previously ignored. Layer and
Theme/ThemeUpdate gain new public fields, so struct literals without
..Default::default() need them added.

Added

• SVG data marks carry a stable class: every element a layer draws gets
  class="gg-mark gg-<kind>" (gg-point, gg-line, gg-band, gg-bar,
  gg-text, by primitive) and data-geom="<geom name>", so a host can select
  marks without matching grid lines, axes or legends, which stay unclassed
  (#121). While a mark part is set (e.g. an arrow head), the part is the
  effective kind, so arrow heads carry gg-mark gg-arrow rather than their
  parent's gg-band/gg-line. data-geom on reference lines reads hline,
  vline or abline. New DrawBackend::set_mark_geom hook (default no-op),
  forwarded by compose::Measure as well as the panel backend adapter, so
  PlotGrid marks are classed too.
• New default Geom::keeps_na_as_break hook (default false; GeomLine/
  GeomPath/GeomStep return true): a row whose position is non-finite is
  no longer deleted from the built data for these geoms, so draw_line's
  per-vertex split (below) actually has a gap to split on, instead of the
  row being spliced out and its finite neighbours silently joined across it.
• GGPlot::svg_attr(name, value)/svg_attrs(iter) — and the same API on
  PlotGrid — add caller attributes to the root <svg>: data-*/aria-*
  (the same charset-restricted suffix Aes::data_attr uses), or the fixed
  names role/id/class/lang. Anything else, or a name ggplot-rs
  already writes to the same root, is dropped with a build warning rather
  than silently overwriting or duplicating it.
• GGPlot::svg_warnings_attr(bool) (and on PlotGrid) emits data-warnings
  as a JSON array of the build warnings, [] when there are none.
• Per-mark formatted values: new default Scale::format_value(&Value) -> Option<String>, overridden by ScaleContinuous (with_value_formatter,
  falling back to the label formatter then the locale, and inverting the
  scale's own transform first — scale_y_log10's data-fvalue shows
  "100", not the transformed-space "2"), ScaleDateTime (with_date_ labels, else the locale date format, else plain ISO — the same order
  the axis break labels already use), and the size/linewidth/colour-
  continuous scales (their own formatter). GGPlot::svg_formatted_values (bool) (default off) turns on data-fvalue (the value aesthetic's
  scale — y for most geoms, fill for hex/tile/bin2d) and data-fx (the x
  scale, written only when it differs from the plain data-x) on marks.
  New DrawBackend::wants_formatted_values/set_mark_fvalue/
  set_mark_fx hooks (all default no-ops). A vline/hline's data-x and
  tooltip show the matching scale's formatted value too (e.g. an ISO date
  on a datetime x) when the flag is on.
• GGPlot::tooltip(template): a {column} tooltip template for the most
  recently added layer, replacing its default <title>. Each placeholder
  is filled from the built (post-stat) layer data — so it can reference
  a stat-computed column, e.g. a hex's count — formatted through the
  column's scale when it has one. An absent column renders empty, not
  literally {column}; {{/}} are literal braces.
• Geom text inherits the theme font: GeomText/GeomLabel/GeomTextRepel/
  GeomLabelRepel/GeomBracket/text annotations resolve their font family
  as the geom's own fontfamily (previously a dead field on GeomText/
  GeomLabel), else theme.geom.family, else theme.text.family, else
  "sans-serif".
• #[non_exhaustive] ElementGeom { ink, paper, accent, family } (ggplot2
  4.0's element_geom), plus Theme.geom, ThemeUpdate.geom,
  Theme::set_geom/with_ink_paper, and ThemeConfig's geom.ink/
  geom.paper/geom.accent/geom.family config keys. A new default
  Geom::apply_element_geom (a no-op) is called once per layer before the
  brand/primary colour, and only for a layer not configured explicitly
  via geom_*_with(...) (the same explicit_style gate the brand colour
  already uses — review PR#141 finding 4: a field-value comparison
  against the hard-coded default can't tell an explicitly-set colour that
  happens to equal it apart from one nobody touched, so the gate skips the
  call entirely instead). Covered geoms: GeomPoint/GeomLine/
  GeomPath/GeomText/GeomLabel/GeomBoxplot recolour their built-in
  black/(50, 50, 50)/white defaults from ink/paper, and GeomSmooth
  from accent. The legend key swatch colour is now
  theme.geom.ink.unwrap_or((50, 50, 50)). Every other geom (segment,
  curve, reference lines, errorbar/linerange/pointrange, step, rect/tile
  outlines, density/violin/area outlines, crossbar) does not yet honour
  ElementGeom — tracked at #142.
• ScaleColorContinuous/ScaleColorGradient2/ScaleColorGradientN gain
  with_limits(lo, hi), fixing the colour domain (not just the legend) to
  a range independent of the trained data — e.g. [-1, 1] for a
  correlation matrix. New Oob enum (Censor, the default: an
  out-of-range value renders in the scale's na_value; Squish, opt-in
  via with_oob: clamped to the nearest limit).
• The same three scales gain na_value (default (127, 127, 127),
  ggplot2's "grey50") and a with_na_value builder: a genuinely missing
  (NA) or non-finite value, and a value censored by Oob::Censor, all
  render in na_value instead of a bare None the geom silently papers
  over with its own unrelated default colour.
• New default Geom::consumes_tooltips hook (default false; the 13
  geoms that actually read a per-row tooltip — bar/col/histogram/hex/
  line/path/point/polygon/rect/sf/segment/curve/tile — return true):
  GGPlot::tooltip(template) on a layer whose geom ignores tooltips now
  pushes a build warning instead of silently having no effect.

Fixed

• Scale::format_value on ScaleContinuous now inverts the scale's own
  transform (scale_x_log10, etc.) before formatting: data-fvalue and a
  {y} tooltip-template placeholder on a log-scaled y = 100 showed "2"
  (the transformed-space value the built data actually holds) instead of
  "100", while the axis labels correctly showed the inverse already.
• ScaleDateTime::format_value now honours with_date_labels (falling
  back to the locale date format, then plain ISO) instead of always
  ISO/locale regardless of it — the axis break labels already did. This
  also un-breaks data-fx on a datetime x with with_date_labels set: it
  was suppressed as "equal to data-x" because the un-formatted ISO
  fallback happened to be the only thing compared.
• PlotGrid cells each honour their own svg_formatted_values flag
  (previously only a top-level GGPlot::render_svg_native* call applied
  it; a plot composed into a grid lost data-fvalue/data-fx even with
  the flag set on that cell).
• ScaleColorContinuous/ScaleColorGradient2/ScaleColorGradientN's
  with_limits(lo, hi) with lo > hi (e.g. a typo) no longer panics
  under Oob::Squish (f64::clamp panics when min > max) — every
  value is treated as censored (na_value) regardless of the configured
  Oob, same as Oob::Censor already did, and Scale::validate now
  pushes a build warning naming the inverted limits.
• The root/PlotGrid svg_attr collision warning now distinguishes a
  name ggplot-rs itself writes (data-plot, data-grid, …) from a
  second svg_attr/svg_attrs call for the same name — the latter
  previously said "collides with an attribute ggplot-rs already writes
  to the root", which wrongly blamed ggplot-rs for the caller's own
  earlier call; it now says "was already set by an earlier svg_attr
  call".

Changed

• Missing colour values render grey on continuous colour/fill scales (na_value, default grey50, as in ggplot2), also without limits. Previously NaN rendered black and NA the low end of the gradient.
• fill-opacity/stroke-opacity is written only when the value isn't
  1.000 (the SVG default); it was previously written unconditionally on
  every mark.
• draw_line splits at a non-finite vertex into separate <polyline>
  marks instead of silently dropping the bad vertex and joining its finite
  neighbours across the gap (ggplot2's NA-break semantics) — for
  geom_line/geom_path/geom_step only (see keeps_na_as_break above).
  A run shorter than 2 points is dropped, the same as ggplot2 drops a
  one-observation path piece, rather than becoming an invisible 1-point
  polyline mark. GeomStep clamps only an infinite pixel coordinate to
  the panel edge (stat_ecdf's ±Inf padding); a NaN coordinate is left
  alone so it breaks the line instead of being clamped to an edge it was
  never meant to reach.
• "geom", "fvalue" and "fx" join "part" as reserved data_attr
  names (alongside the attributes they already name): a mapping under any
  of these is rejected with a build warning, the same as "part" already
  was, so no element can ever carry two data-geom/data-fvalue/data-fx
  attributes.

Breaking

• Layer gains a public tooltip_template: Option<String> field.
• Theme gains a public geom: ElementGeom field; ThemeUpdate gains a
  public geom: Option<ElementGeom> field. Struct literals for either
  without ..Default::default() need the new field added.