Follow-ups to #366: dark legacy tile override, legacy attribution, zoom cap #390

Follow-ups from the review of #366 (#364, merged).

1. Dark legacy override is still masked. tiles.dark / map.tiles.darkUrl is overridden by the registry default the same way the light override was before fix(maps): preserve legacy light tile URL overrides (#364) #366. No legacy branch exists for dark. Mirror the fix(maps): preserve legacy light tile URL overrides (#364) #366 precedence (explicit choice > configured default > legacy URL > built-in fallback) for dark.
2. Attribution for legacy raw URLs. A legacy URL carries no vendor metadata, so e.g. an OpenTopoMap URL gets the generic © OpenStreetMap contributors credit (missing CC-BY-SA/SRTM). Consider allowing an attribution override alongside the legacy URL, or document it.
3. Zoom cap. The legacy-URL spec hard-codes maxZoom 18 (osm-standard), so a custom provider that supports z19 is clipped one level. Consider a configurable max zoom.