statgl_plot: series_tags, general two-group, named palette

[emilmalta]

Three additive changes driven by the climate-page station-filter
use case in statwww:

• New series_tags arg attaches per-series metadata as series.options.tags[], for downstream JS (e.g. the statglshortcodes filter shortcode) to read without relying on series-name matching. For two-group charts, tags are keyed by the g1 value so a single station filter matches both the max and min series for that station in one click.

• group = c(g1, g2) now works outside pyramid mode as the general "groups by g1, colour by g2" overlay: g1 splits series, g2 drives colour and the legend (one entry per g2, series sharing a g2 are linkedTo each other). Replaces the manual two-plot palette splice for min/max-style overlays. Pyramid two-group behaviour is functionally unchanged.

• palette accepts a named char vec for two-group charts, mapping g2 -> colour directly: palette = c(Maks = "#fa8b2a", Min = "#2caffe") The single-name and unnamed-vec forms still ramp across n_g2.

Behaviour changes worth flagging:

• Pyramid two-group: the legend representative is now the first g1 level rather than the second. Labels, colours, and click-to-toggle behaviour are identical; only the internal linkedTo direction differs.

• The auto legend title (g2 column name shown as a bold header above legend entries) is removed across the board. It read as raw column noise on non-pyramid two-group charts. Add a title explicitly via hc_legend(title = list(text = "...")) if you
  want one.

Each two-group series now also carries g1_label internally so the tooltip and series_tags can key off it instead of re-parsing series.name (which has been renamed to g2).