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/testsprite2.json

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