Fix varregion scene capture and legacy rendering

crates/libremetaverse/CHANGES.md

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+# MetaCrate compatibility deviations
+
+This file records intentional differences from the C# LibreMetaverse source. Preserve or
+re-evaluate these changes when importing a newer upstream version.
+
+## OpenSim varregion dimensions
+
+- `GridRegion` retains the optional `MapBlockReply.Size` dimensions. Missing or zero dimensions
+  fall back to 256 x 256 metres.
+- `Simulator::region_size()` exposes dimensions resolved after login and is marked `must_use`,
+  while the mapped public `size_x` and `size_y` fields remain unchanged for API compatibility.
+- A matching map-region event updates the current simulator's resolved dimensions. Movement,
+  border prediction, terrain lookup, and terrain archive export use the resolved dimensions.
+- The cache distinguishes explicit `Size` blocks from the 256 x 256 fallback. Startup waits for
+  an explicit size until timeout, and a later fallback reply cannot downgrade a resolved size.
+- Region lookup no longer synthesizes the current simulator as a 256 x 256 `GridRegion`; it
+  requests the authoritative map block.
+
+Files: `src/grid_manager.rs`, `src/network_manager.rs`, `src/agent_movement.rs`,
+`src/asset_archive.rs`.
+
+## Asynchronous region-reply correlation
+
+- Name and handle lookups subscribe to the continuously dispatched `GridRegion` event stream
+  before sending a request.
+- Each lookup accepts only the matching name or handle. Unrelated grid, object, avatar, or map
+  traffic cannot complete the request early.
+- Cancellation and the mapped map-request timeout still bound the one-shot waiter.
+- The default map-request timeout is 15 seconds rather than 5 seconds because OpenSim map
+  replies on the live varregion can arrive after 7-8 seconds during login.
+
+Files: `src/grid_manager.rs`, `src/client_core.rs`.
+
+## OpenSim asset transport fallback
+
+- When the OpenSim asset capability returns no payload for an asset, the request falls back to
+  the existing UDP asset transfer path instead of reporting a missing asset immediately.
+
+File: `src/asset_manager.rs`.