Basemap providers and settings UX: design evaluation

[MaskyS]

Design doc for #28. Evaluates where basemap configuration and choice should live (depot settings / map toolbar / graph-authored blocks / hybrid) and proposes a provider-aware model referenced against MapLibre's native primitives.

All provider claims are sourced from provider documentation (URLs inline); MapLibre claims from maplibre.org; code claims carry file:line references to the current tree.

Highlights:

• Recommended hybrid: keys and provider enablement in depot settings, basemap choice in the map toolbar with preview → Save, graph-wide default setting.
• Declarative provider adapters (credential slot + curated style list + policy metadata) replacing the bespoke MapTiler shortcut; no provider on the shortlist needs transformRequest, and none offers style enumeration, so style lists are curated data.
• Provider tiers driven by key restrictability (settings sync graph-wide): OpenFreeMap keyless; MapTiler / Esri / Protomaps first-class keyed; Stadia / Thunderforest with explicit warnings; Jawg / CARTO excluded with reasons.
• Migration guard called out as step 1: provider normalization strips unknown fields today, so older builds would destroy a per-provider styles list on write.

Proposal only — no implementation in this PR.