Kestrel coastal terrain

Adapted from ZeroMind's dragon_terrain material (ac829913-ea3e-4dab-9318-7b4e43d324a3). Uses terrainBlend with scrub, grass and exposed-rock bands adjusted to Kestrel Island's elevation. Authored in current properties syntax so all tint and altitude fields reach the shader. The mission director generates its terrain mesh and collision height field.