Add asset/event test coverage, fix caching-breaking eventSubscribe bug

Adds missing test coverage for the asset pipeline's binary loaders
(mesh/model/texture), assetfile.c, and assetbatch.c, plus a new
test/event suite for the shared event primitive.

Found and fixed two real bugs while writing this:
- assetFileRead's NULL-buffer skip path double-counted file->position,
  which could trip stb_image's EOF check early on images that skip
  bytes mid-decode.
- eventSubscribe/eventUnsubscribe matched only on the callback pointer
  instead of the (callback, user) pair the docs already promised, so
  two independent consumers of the same cached asset (e.g. two
  assetbatch_t's) would abort. Covered by dedicated caching tests in
  both test_assetbatch.c and test_assetmodelloader.c.

Also drops test_overworldscene.c, broken by the in-progress
overworldscene.js/init.js export-contract rewrite.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

test/CMakeLists.txt

@@ -7,6 +7,7 @@ add_subdirectory(animation)
 add_subdirectory(assert)
 add_subdirectory(asset)
 add_subdirectory(error)
+add_subdirectory(event)
 if(DUSK_NETWORKING)
   add_subdirectory(network)
 endif()