step 4: add async tiles and persistent cache

README.md

@@ -2,7 +2,7 @@
 
 `dpg-map` is a Dear PyGui map widget package under rebuild.
 
-The Step 3 widget shell is in place:
+The Step 4 tile manager is in place:
 
 ```python
 import dpg_map as dpgm
@@ -28,15 +28,16 @@ Implemented so far:
 - thread-safe logical map state and map registry
 - command queue with coalescing for overlay and view updates
 - logical marker, polyline, trajectory, and layer models
-- persistent disk cache path, metadata, scanning, and prune planning
+- persistent disk cache paths, metadata, scanning, pruning, and clearing
 - Dear PyGui `child_window` + measured-size `drawlist` widget shell
-- GUI-thread frame pump that drains commands and redraws a placeholder background
+- GUI-thread frame pump that drains commands, manages textures, and draws raster tiles
 - sizing helpers that preserve the last non-zero size across hidden layouts
 - interaction hit-rectangle calculation for the measured map area
+- asynchronous tile workers that read disk cache, fetch HTTP, and decode images
+- memory cache with visible-tile protection and GUI-thread texture deletion
 
-Real tile loading and overlay drawing are not implemented yet. Step 4 will add
-the asynchronous tile manager, persistent tile loading, and GUI-thread texture
-creation.
+Overlay drawing is not implemented yet. Step 5 will add pan/zoom interaction and
+view command projection.
 
 Examples:
 
@@ -46,4 +47,5 @@ uv run python examples/sizing_window.py
 uv run python examples/sizing_child.py
 uv run python examples/sizing_table.py
 uv run python examples/hidden_tab.py
+uv run python examples/cache_stress.py
 ```