Add physics, heap, json_helpers, game, and actor test suites

Raise line coverage from 39.6 to 61.8 percent with four new suites and an
extension to the actor suite, and register every suite through a single
CMake list so a new test file cannot be left out of the coverage fixture.
Give the test targets a build-tree RPATH and prepend the build tree to
LD_LIBRARY_PATH for CTest, so a developer with a previously installed
libakgl.so exercises the library that was just compiled.

Fix six defects the new tests exposed:

- akgl_physics_simulate read self->gravity_time before its NULL check, so a
  NULL backend crashed instead of reporting AKERR_NULLPOINTER.
- akgl_game_save transposed CLEANUP and PROCESS, which placed the fclose
  inside the PROCESS switch. An ordinary save never flushed or closed its
  stream and produced an empty file.
- akgl_game_save_actors wrote each name table terminator from the address of
  a single char, emitting stack contents into the save file and a sentinel
  the loader could not recognize.
- akgl_game_load_objectnamemap used CATCH directly inside while(1), where the
  break leaves the loop rather than propagating, so a truncated name table
  loaded as a successful game.
- akgl_Actor_cmhf_up_on and _down_on dereferenced actor->basechar with no
  NULL check, unlike their left and right counterparts.
- akgl_actor_logic_movement checked actor twice instead of checking
  actor->basechar before dereferencing it.

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

src/physics.c

@@ -128,13 +128,18 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_simulate(akgl_PhysicsBackend *se
 	.flags = 0,
 	.layerid = 0
     };
-    SDL_Time curtime = SDL_GetTicksNS();
-    float32_t dt = (float32_t)(curtime - self->gravity_time) / (float32_t)AKGL_TIME_ONESEC_NS;
+    SDL_Time curtime = 0;
+    float32_t dt = 0;
     akgl_Actor *actor = NULL;
 
     FAIL_ZERO_RETURN(e, self, AKERR_NULLPOINTER, "self");
     FAIL_ZERO_RETURN(e, self->move, AKERR_NULLPOINTER, "self->move");
 
+    // Reading the elapsed time requires self, so it cannot be hoisted above
+    // the null check.
+    curtime = SDL_GetTicksNS();
+    dt = (float32_t)(curtime - self->gravity_time) / (float32_t)AKGL_TIME_ONESEC_NS;
+
     if ( opflags == NULL ) {
 	opflags = &defflags;
     }