v0.2.0

Adds PNG map snapshots (basemap + overlays) — additive, non-breaking.

• MapAdapter.snapshot(opts?: SnapshotOptions): Promise<Blob> — always resolves to
  an image/png Blob. scale (output pixel-ratio) defaults to
  window.devicePixelRatio, i.e. captures "as on screen".
• MapLibre and OpenLayers are supported. Capture happens inside the engine's
  render frame, so the host map needs no special flag (no preserveDrawingBuffer).
• Leaflet is not supported yet: snapshot() rejects (tiles are <img>, overlays
  are SVG/DOM — no single exportable canvas). A DOM-snapshot approach is planned.
• Toolbar: new ToolbarOptions.snapshot preset — "none" | "low" | "native" | "medium" | "high" (default "native"). When set, addToolbar adds a camera button wired to
  download a PNG; on Leaflet the button is shown but disabled (the reason is its
  tooltip). Preset → output pixel-ratio via the exported snapshotScale():
  low → 1, native → devicePixelRatio, medium → 2, high → 3. medium/high
  are supersampling — best-effort (a re-scale, not extra map detail).
• New exports: snapshotScale, downloadPng, SNAPSHOT_ICON_SVG, and the
  SnapshotOptions / SnapshotLevel types. ToolbarItem gains an optional disabled.