Extends the Loaded result record with a LandblockStreamTier discriminator
and a LandblockMeshData payload (default! stub — T13 wires the real
off-thread mesh build). Adds the Promoted variant for Far→Near upgrades
that only need the entity layer, not a mesh rebuild.
LandblockStreamer.HandleJob passes Tier.Near + default! MeshData at the
existing synchronous load site; StreamingControllerTests updated to
match the new positional signature.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Called once per frame from OnUpdate. Owns a StreamingRegion and uses
delegates into LandblockStreamer + a terrain-apply callback so unit
tests can inject fakes. Handles first-tick bootstrap (whole window
loads), boundary recenter (diff against previous center), and
drain completions (up to N per frame to cap GPU upload spikes).
4 new tests, all green.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
