Adds TwoTierDiff — the five-list output of StreamingRegion.RecenterTo
(ToLoadFar/Near, ToPromote, ToDemote, ToUnload) per spec §4.2. Used by
T3–T6 (StreamingRegion) and T13 (StreamingController).
Extends LandblockStreamJob.Load with a LandblockStreamJobKind parameter
so the streaming worker can route far vs near vs promote jobs differently
(spec §4.3). Patches the one call site in LandblockStreamer.EnqueueLoad
with LoadNear as a placeholder that preserves today's full-load semantics
until T11 activates the worker thread and T16 routes by tier.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
