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/copy.cmake

@@ -0,0 +1,22 @@
+# Copyright (c) 2026 Dominic Masters
+#
+# This software is released under the MIT License.
+# https://opensource.org/licenses/MIT
+
+# Handler that copies an asset into the built assets directory as is. Every
+# handler has the signature (SOURCE RELATIVE OUT_VAR) and must set OUT_VAR, in
+# the caller's scope, to the file it produces.
+function(dusk_asset_copy ASSET_SOURCE ASSET_RELATIVE OUT_VAR)
+  set(ASSET_OUTPUT "${DUSK_BUILT_ASSETS_DIR}/${ASSET_RELATIVE}")
+
+  add_custom_command(
+    OUTPUT "${ASSET_OUTPUT}"
+    COMMAND ${CMAKE_COMMAND} -E copy_if_different
+      "${ASSET_SOURCE}" "${ASSET_OUTPUT}"
+    DEPENDS "${ASSET_SOURCE}"
+    COMMENT "Copying asset ${ASSET_RELATIVE}"
+    VERBATIM
+  )
+
+  set(${OUT_VAR} "${ASSET_OUTPUT}" PARENT_SCOPE)
+endfunction()