Repair embedded dependency build and test setup

Isolate vendored test registration, namespace project error codes, and update dependency revisions. Fix mutable sprite path handling, out-of-tree fixtures, and charviewer initialization while documenting the outstanding character-to-sprite refcount bug.

Co-authored-by: Codex (GPT-5) <noreply@openai.com>

tests/assets/testcharacter.json

@@ -1,7 +1,8 @@
 {
     "name": "testcharacter",
-    "velocity_x": 0.20,
-    "velocity_y": 0.20,
+    "movementspeed": 1,
+    "acceleration_x": 0.20,
+    "acceleration_y": 0.20,
     "sprite_mappings": [
 	{
 	    "state": [
@@ -12,7 +13,7 @@
 	},
 	{
 	    "state": [
-		"ACTOR_STATE_DEAD"
+		"AKGL_ACTOR_STATE_DEAD"
 	    ],
 	    "sprite": "testsprite2"
 	}

tests/assets/testsprite.json

@@ -1,6 +1,6 @@
 {
     "spritesheet": {
-	"filename": "assets/spritesheet.png",
+	"filename": "spritesheet.png",
 	"frame_width": 48,
 	"frame_height": 48
     },

tests/assets/testsprite2.json

@@ -1,6 +1,6 @@
 {
     "spritesheet": {
-	"filename": "assets/spritesheet.png",
+	"filename": "spritesheet.png",
 	"frame_width": 48,
 	"frame_height": 48
     },