chartlet 0.2.0

Adds stacked areas, points and range groups, exact value axes and the stylesheet in parts. The
specification keeps schemaVersion: 1; every specification that rendered with 0.1.0 still
renders, with the changes in output listed under Changed and Fixed.

Added

• Time charts: "mark": "point" draws a dot per observation and no line, as in a scatter plot.
• Time charts: "stack": "normal" on a pane stacks its area layers; the crosshair of the
  interactive module places stacked values on top of the stack.
• Time charts: "tooltips": "markers" keeps tooltips on drawn markers only, so dense lines stay
  small.
• Time charts: months ("2026-03") as times, and timeAxis.precision (year, month, day,
  minute) for how tooltips, the table and the description write times.
• Range bars: group per range, drawn in a palette color with a legend entry per group.
• Value axis: step sets the tick interval and exact keeps min and max as the ends.
• The shared stylesheet in parts: stylesheet_common() and stylesheet_types(),
  stylesheet({ types: [] }) and stylesheet({ types, common: false }), chartlet stylesheet --common and --no-common. The common part comes first; a type's part styles only charts of
  that type.

Changed

• More than 16 categories on a horizontal axis whose labels do not fit: every n-th category is
  labelled, with the room of n bands, and the warning labels_thinned.
• Charts narrower than 480 pixels draw their title at 18 pixels.
• In the shared stylesheet, each type's rules now come together, type by type.

Fixed

• The markers of the first series in a time chart with several layers take its palette color.
• A horizontal range bar chart keeps room for its last tick label.