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>

TODO.md

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+# TODO
+
+1. **Make character-to-sprite state bindings release their references symmetrically.**
+   `akgl_character_sprite_add()` increments the sprite refcount for every state-map
+   binding, but no corresponding removal API exists. Replacing a state binding
+   also leaves the previous sprite's refcount incremented. When
+   `akgl_heap_release_character()` drops the character refcount to zero, it clears
+   the character registry entry and zeroes the structure without enumerating
+   `state_sprites`, decrementing the bound sprites, or destroying the SDL property
+   map. Implement binding removal/replacement so each removed binding releases
+   exactly one sprite reference. On final character release, enumerate every
+   remaining binding (the existing `akgl_character_state_sprites_iterate()` release
+   path may be reusable), release each reference, destroy `state_sprites`, and then
+   clear the character. Add tests for removal, replacement, duplicate sprite
+   bindings across multiple states, and final character release.