feat(game): add survivor_game with arena and quarter-view camera

Static arena with 20x20 floor and 4 box obstacles rendered using the
forward PBR pipeline with shadow mapping. Fixed quarter-view camera
at (0, 15, 10) looking at origin.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -24,6 +24,7 @@ members = [
     "crates/voltex_script",
     "crates/voltex_editor",
     "examples/editor_demo",
+    "examples/survivor_game",
 ]
 
 [workspace.dependencies]