Widens LandblockMesh.Build's surfaceCache parameter from Dictionary to
IDictionary so any IDictionary implementation compiles at call sites.
Switches GameWindow._surfaceCache from Dictionary to ConcurrentDictionary
so T11's streaming worker can call Build off the render thread without
a lock.
The TryGetValue+assign lookup inside Build is not atomic, but BuildSurface
is deterministic (same palCode -> same SurfaceInfo), making last-write-wins
under concurrent access benign. Comment added at the pattern site.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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