`terrain_island(cx, cz, r, fall)` keeps land out to `r` and then scales the
terrain down into the water over `fall` metres and on to a shelf. Scaling rather
than blending to a fixed bed is what makes the coastline come out of the terrain
that is already there: low ground turns into beach and shallows, high ground
into cliff. `r = 0` leaves the survey alone, so nothing that does not ask for an
island is touched.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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