[sandesh-sp]

Closes #570

Summary

• ScaleBar tool (src/essence/Tools/ScaleBar/) shows a metric or imperial scale bar, by default in a float-bottom-left panel. The bar is labelled with a round distance (1, 2, 5, 10, …) and switches between m/km or ft/mi.
• Scale under the pointer. The bar gives the scale where the pointer is on the map, and at the map's centre while the pointer is off the map (or over empty space past a zoomed-out Web Mercator world). A scale at one point holds at any zoom and tilt, so the bar always shows.
• Measured on screen, for any engine. mapScale.ts unprojects two points a few pixels apart and measures the great-circle distance between them on the mission's planet. It works for deck.gl and Leaflet alike, and for non-Mercator projections.
• Bus API: map:getMetersPerPixel takes an optional { x, y } map container point and returns ground metres per CSS pixel there (the map's centre when omitted). map:mouseout reports the pointer leaving the map; deck.gl raises it on the container's mouseleave, Leaflet natively. Documented in Event-Bus-API.md.
• Exports: while the bar is on screen, Share's PNG and PDF exports draw a matching bar at the same spot, measured there. map:getScreenshot accepts { scaleAt } and returns metersPerPixel in image pixels plus the scaleAnchor, read before the capture starts, so the bar is correct on high-DPI displays.
• Refresh: the tool reads the scale on handler-ready, every map:move and map:moveend, pointer moves, and window resize (once per frame). Reads that would draw the same bar skip the render, and stale reads that settle out of order are dropped.
• deck.gl emits move on every camera frame: overlay mode, standalone mode, and side-by-side pane drags. map:move subscribers and addOverlay's reposition listener now track the camera during a gesture. Projections share one viewport per camera.

Behaviour and limits

• Exports keep view-wide rules, since an image cannot show where the scale applies: no bar when the map is tilted more than 10°, or when the scale varies more than 1.5× down the view (sampled on five rows). The second rule hides the exported bar at continent- and world-wide views (about zoom ≤ 2 at the equator, ≤ 4 at mid-latitudes on an 800×600 map) and never lets it return as the map zooms out further.
• No bar when the map has no size yet or the planet radius is not configured.
• Exports draw a bar exactly when the tool's bar is on screen (plugin:scalebar:getSettings). Making this a plugin-neutral contract is deferred to the plugin-to-plugin communication work.
• Lifting the bottom float zones above the basemap's logo and attribution is split out to a follow-up branch (feature/570-map-chrome-insets), since it changes every bottom float panel. Until it lands, a bottom-left bar can sit close to the MapLibre logo.

Test plan

• npx vitest run: 215 files, 3,666 tests pass
• Move the pointer from the bottom to the top of a deck.gl and a Leaflet map and check the bar's label follows the latitude under it
• Move the pointer off the map, and over empty space above a zoomed-out world, and check the bar shows the centre's scale
• Scroll-zoom with the pointer still and check the bar keeps up frame by frame
• Tilt the map past 10° and check the bar still shows, while a PNG export carries none
• Zoom out to a world view, export PNG, and check the export has no bar; zoom in and check it returns
• Drag a side-by-side comparison pane and check the bar updates during the drag
• Export PNG and PDF on a high-DPI display and check the exported bar matches the on-screen bar and sits at the same spot
• Check HTML overlays (addOverlay) still track the camera while panning in overlay mode