vellumplot 0.2.1

vellumplot 0.2.1

Consumes vellum's new compound native + mm unit (requires vellum >= 0.1.1).

• Label nudges: mark_text() / mark_label() gain nudge_x / nudge_y
  (in millimetres, +x right / +y up) that shift a label by an exact absolute
  distance, device-resolved so the nudge is constant regardless of scale or panel
  aspect.
• Device-exact drop shadows: shadow()'s x / y offset is now an absolute
  distance in millimetres (was a panel-relative npc fraction), so a drop
  shadow stays the same physical distance and is isotropic on non-square panels.
  Defaults changed accordingly (a small down-right drop). Replaces the previous
  npc-fraction workaround.

Device-space dodge (mark_bar) and jitter (mark_point) remain data-space for
now; converting them to the compound unit is deferred (it changes existing
rendered output and needs snapshot review).