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>

tests/assets/snippets/test_json_helpers.json

@@ -0,0 +1,19 @@
+{
+    "name": "json helper fixture",
+    "count": 42,
+    "ratio": 2.5,
+    "negative": -17,
+    "enabled": true,
+    "disabled": false,
+    "nested": {
+        "inner": "value",
+        "innercount": 7
+    },
+    "integers": [10, 20, 30],
+    "strings": ["alpha", "beta"],
+    "objects": [
+        { "id": 1 },
+        { "id": 2 }
+    ],
+    "mixed": [1, "two", { "three": 3 }]
+}