Find a street intersection

Resolves the city anchor, loads the street walk graph around its center
(the city's extent, up to 5 km from its center), and finds the junctions
inside it where an edge of one street meets an edge of the other.

Street names match case-insensitively and through the same USPS abbreviation
set as geocode_address (Avenue/Ave, Parkway/Pkwy, West/W, NW/Northwest,
5th/Fifth), plus a directional suffix the caller left off ("Pennsylvania
Avenue" finds "Pennsylvania Avenue NW").

Returns {"results": [{"lat": ..., "lon": ..., "streets": [name_a, name_b]}, ...],
"anchor": {"name": ..., "id": ..., "country": ..., "admin_context": [...]},
"note": ...}. `streets` is the map's own spelling of the two streets at that
crossing, in street_a/street_b order — not an echo of the inputs. When
multiple crossings exist, results are ordered nearest to the city center
first and capped at 5. If one or both streets do not resolve in the city,
returns empty results and a note naming the unresolved street; "truncated":
true means the street graph hit its size cap and may be missing some.