Include actor.h from controller.h so the header compiles on its own

controller.h declares three handler pointers taking an akgl_Actor * and
included nothing that declares that type, so any translation unit reaching
for it before akgl/actor.h failed with "unknown type name 'akgl_Actor'".
src/controller.c never noticed because it includes akgl/game.h first, and
tests/controller.c carried a comment explaining the workaround.

Included rather than forward-declared: akgl_Actor is a typedef of a named
struct and repeating a typedef is C11, not C99. It closes no cycle --
actor.h reaches only types.h and character.h.

tests/headers.c is a whole suite for one #include on purpose. A second
#include after the first proves nothing about the second, because by then
the first has dragged its dependencies in; covering another header means
another file shaped like this one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -188,6 +188,7 @@ set(AKGL_TEST_SUITES
   draw
   error
   game
+  headers
   heap
   json_helpers
   physics