Address→Hex Lookup: Slice 5 (optional) — render active global holds on the map

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

Goal: Overlay active global holds as a distinct Mapbox layer on /map and /explorer, sourced from hold:index.

Acceptance: held global cells appear visually distinct from curated-200 inventory and from available cells.