vellumwidget 0.7.0

• Bug fix: hover snapped to the wrong mark on a scaled plot. When the svg was
  rendered at a different size than its viewBox implies (the widget's
  max-width:100%, the RStudio Viewer pane, an embedded/saveWidget page), hover
  highlighted a mark offset from the cursor (usually up-and-left); click was
  unaffected because it uses the DOM target. The client→scene hit-test mapping
  relied on getScreenCTM() / viewBox math, which can disagree with how the
  content is actually laid out when the svg carries width/height attributes.
  Hit-testing (hover, brush, lasso) now calibrates the transform from the real
  rendered positions of the marks themselves versus their known scene bboxes —
  exact under any viewBox / attribute / CSS-scale / zoom combination. Tooltip
  anchoring uses the mark's own rendered position.

• Breaking: interaction is now declared in the plot, not on as_widget().
  The interaction-intent arguments have been removed — hover_color,
  selected_color, dim_opacity (now vellumplot::condition()), and the
  hover/select/brush/lasso/tooltip/nearest/zoom/axis_zoom/
  zoom_marks toggles (now on by default — interactive-by-default). The
  signature drops from ~36 to 24 arguments. Per-plot interaction (highlight,
  filter, cross-filter) travels with the spec via
  vellumplot::select_point()/select_interval()/condition()/filter_by().
  Requires vellumplot (>= 0.6.0.9000). (hover_mode, crosshair,
  legend_click, select_mode, and the tooltip-appearance args remain until
  their spec equivalents land.)

• Declarative interactivity: cross-view filtering. A selection defined on one
  cell of a composition and referenced by filter_by() on another now
  cross-filters — brush/select the source cell and the linked cell narrows to the
  matching rows, matched by a cross-view join id. The source cell stays fully
  visible; only the target cell hides (scoped by per-cell-unique keys). All in
  one widget runtime, no server round-trip.

• Declarative interactivity: filters. A vellumplot plot that declares
  filter_by(selection) now hides everything outside the selection when that
  selection's gesture fires — reusing the cross-filter path. Driven entirely by
  the plot spec (no as_widget() argument). Single-view for now; the cross-view
  form (brush one panel, filter another) follows.

• Declarative interactivity: conditional encodings. as_widget() now reads
  the interaction model a vellumplot plot declares (select_point() /
  condition()) off the compiled spec and enacts it on the frozen scene. A
  hover- or click-driven selection with a condition() on an aesthetic keeps
  selection members at their if_true style and applies if_false to
  non-members (an explicit colour, or the theme dim as a spotlight) — reverting
  when the selection empties. No new as_widget() argument: the behaviour
  travels with the plot. (Filters, cross-view, and scale binds follow.)