Add NearRadius/FarRadius properties and a four-arg constructor
(centerX, centerY, nearRadius, farRadius). Radius is set to farRadius
so existing hysteresis math (unload threshold = Radius+2) uses the
outer ring as the bookkeeping boundary. Old three-arg constructor
becomes a thin wrapper: this(cx, cy, radius, radius) — no behaviour
change, 25 pre-existing streaming tests still pass.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
