Compute an approximate concave hull using alpha shape (simplified).

Arguments

• coords: Flat Float64Array [lng0, lat0, lng1, lat1, ...].
• alpha: Controls concavity. Larger values → more convex (α → ∞ gives convex hull). Smaller values → more concave. Typical range: 0.1–10.0.

Returns

Flat Float64Array of concave hull vertices (closed: first == last).