Background thread pulls load/unload jobs from an inbox channel, invokes
a caller-supplied Func<uint, LoadedLandblock?> (production wraps
LandblockLoader.Load, tests inject a fake), and posts results to an
outbox channel the render thread drains. Graceful shutdown via
CancellationToken; failed loads reported rather than retried.
4 new tests, all green.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
