refactor: --mode window is now Vulkan, remove softbuffer fallback

- --mode window = Vulkan renderer (was softbuffer CPU)
- --mode vulkan removed (merged into --mode window)
- softbuffer dependency removed
- log dependency removed (unused)
- Old window.rs (softbuffer CPU renderer) deleted
- Fallback: if Vulkan init fails, falls back to --mode terminal
- 109 tests, 0 failures

Cargo.toml

@@ -10,12 +10,10 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 fontdue = "0.9"
 winit = "0.30"
-softbuffer = "0.4"
-log = "0.4"
 ash = "0.38"
 ash-window = "0.13"
 raw-window-handle = "0.6"
-bytemuck = "1"
+bytemuck = { version = "1", features = ["derive"] }
 
 [profile.release]
 opt-level = 3