Add asset, resource system, mesh formats, color, SDL3 display and example game

Asset directory with CMake add_asset (extension dispatched), plain file asset
access, resource/resource manager with async load thread and text loader,
URI utilities, mesh format helpers, generated color header, and example game
hooks wired through the engine.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

cmake/asset/obj.cmake

@@ -0,0 +1,10 @@
+# Copyright (c) 2026 Dominic Masters
+#
+# This software is released under the MIT License.
+# https://opensource.org/licenses/MIT
+
+# Handler for .obj (Wavefront mesh) assets. Currently just copies the file.
+function(dusk_asset_obj ASSET_SOURCE ASSET_RELATIVE OUT_VAR)
+  dusk_asset_copy("${ASSET_SOURCE}" "${ASSET_RELATIVE}" ASSET_OUTPUT)
+  set(${OUT_VAR} "${ASSET_OUTPUT}" PARENT_SCOPE)
+endfunction()