Configurable legends for viewer-charts

[texodus]

New legends

Legends are now a fully configurable chart element across all charts:

• legend_mode — "sidebar" (default, the existing right-gutter legend), "none", or "floating": a movable panel overlaying the plot.
• Scroll + windowed painting — every legend scrolls on vertical overflow, and only the visible rows are measured and painted, so charts with hundreds of series stay cheap.
• Direct manipulation — sidebar legends resize horizontally by dragging their divider; floating legends resize on all edges/corners and move by their header. Double-clicking a resizable edge resets that dimension to its default. Every gesture persists through plugin_config.
• Floating geometry is anchor-relative: legend_x/legend_y are normalized offsets measured from legend_anchor, so a corner-glued legend stays glued across panel resizes and off-canvas placement is inexpressible.

Signed numeric color domains now pivot symmetrically around zero (colorRangePivot): a column spanning e.g. [-3, 17] colors and labels as [-17, 17], so hue encodes sign consistently in the glyphs (line/scatter) and the gradient legend agrees with them.

Map tile sources

• Provider definitions moved out of TypeScript into map/tile-sources.json; the code is a generic registry (TileSourceRegistry) over TileSourceSpec entries (id, label, template, subdomains?, attribution, tile_size?, max_zoom?). Bundled providers: OpenStreetMap (osm) and VersaTiles satellite (versatiles-satellite). JSON order is the one knob: it defines the settings-panel enum order, the unknown-id fallback, and the default provider (first entry).
• Public API: registerTileSource(spec) / tileSources() (module exports and statics on the plugin element class) add providers at runtime — they join the settings enum and resolve on every current and future map chart. API keys are embedded in the registered template and never enter plugin_config/save(). The resolved spec rides the existing setPluginConfig/init transport messages alongside the config that names it — no separate sync channel, no ordering hazards. Tile-cache identity is content-derived, so re-registering an id with a new URL invalidates cached tiles automatically.
• The CARTO basemaps moved out of the bundle and into the docs site, which registers them through the public API at bootstrap; they double as the worked example on the new guide page (docs/md/how_to/javascript/map_tile_sources.md).

Map numeric axes: numeric_axes

• On (default): standard numeric axes on map charts with tick labels in degrees (122.4°W / 37.77°N), not Mercator meters.
• Off: the plot fills the entire plugin canvas.

New plugin_config keys

Key	Type	Default	Applies to
legend_mode	"sidebar" | "none" | "floating"	"sidebar"	all legend-capable charts
legend_width_px	number 0–512	0 (auto)	sidebar + floating width
legend_height_px	number 48–1024	160	floating panel height
legend_anchor	"top-left" | "top-right" | "bottom-left" | "bottom-right"	"top-right"	floating
legend_x	number 0–1	0	floating; offset from anchor
legend_y	number 0–1	0	floating; offset from anchor
legend_opacity	number 0–1	1	floating background
numeric_axes	boolean	true	map charts only

Fixes #3189