Address→Hex Lookup: Slice 1 — forward geocode + resolve containing Res-4 cell (read-only)

[tylermalin]

Part of Address → Hex Lookup & Reserve (Res 4) — see docs/prd/address-hex-lookup.md.

Goal: A user types an address, and we render the single H3 Res-4 cell that contains it. Read-only — no reservation.

Scope

• Address autocomplete via Mapbox forward geocoding (Search Box / Geocoding v6 forward), reusing NEXT_PUBLIC_MAPBOX_TOKEN.
• Pure resolver: (lat, lng) -> canonical Res-4 H3 index via h3-js latLngToCell(lat, lng, 4).
• Render the cell: fly map to it, draw boundary via cellToBoundary. Reuse HexBoundaryPreview.tsx patterns.

Acceptance

• Given a lat/lng, returns the correct Res-4 index; getResolution(index) === 4.
• Entering a valid address highlights exactly one hex on the map.
• No write paths touched.

TDD anchor: failing unit test for the pure resolver first (address → containing Res-4 cell), then implement; geocoding I/O mocked.