v0.21.0 — map: one value: keyword + route redesign

Changed

• Map syntax — one value: keyword (breaking) — the map chart type's three
  numeric channels collapse into a single value:, rendered per element: region
  shade, POI marker size, edge/route-leg thickness. Replaces score: (regions),
  size: (POIs), and weight: (edges).
• Map routes — origin header + arrow legs (breaking) — a route is now
  route <origin> [style: arc] followed by indented [-label->] destination
  legs, each an edge with its own in-arrow label, value: thickness, and arc
  shape. Replaces the bare-stop list. Fixes two latent bugs: named-stop metadata
  is no longer silently dropped, and a loop-closing leg no longer double-draws the
  origin marker.
• Map legend labels — region-metric / poi-metric / flow-metric replace
  metric / size-metric (one value: keyword now drives three element
  channels that often carry different quantities).
• Map scope — a bare US state postal code resolves to that state
  (poi Portland OR → Oregon; CA = California) and signals US scope.

Removed

• Map — the description: and date: reserved keys (no v1 surface — they now
  raise an unknown-key error instead of silently no-opping) and the
  active-tag score token (the value ramp is the default; select it by its
  legend name).