Function geohash_encode 

pub fn geohash_encode(lng: f64, lat: f64, precision: u8) -> String

Encode a (longitude, latitude) coordinate to a geohash string.

• precision: number of characters (1–12). Default 6 gives ~1.2 km cells.
• Longitude range: [-180, 180]
• Latitude range: [-90, 90]

Returns an empty string if precision is 0.