feat(renderer): add BMP texture loader and GPU texture upload

Implements parse_bmp (24/32-bit uncompressed BMP to RGBA), GpuTexture with
wgpu 28.0 write_texture API (TexelCopyTextureInfo/TexelCopyBufferLayout),
bind_group_layout, white_1x1 fallback, and 3 BMP parser unit tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/voltex_renderer/src/lib.rs

@@ -2,6 +2,7 @@ pub mod gpu;
 pub mod light;
 pub mod obj;
 pub mod pipeline;
+pub mod texture;
 pub mod vertex;
 pub mod mesh;
 pub mod camera;
@@ -10,3 +11,4 @@ pub use gpu::{GpuContext, DEPTH_FORMAT};
 pub use light::{CameraUniform, LightUniform};
 pub use mesh::Mesh;
 pub use camera::{Camera, FpsController};
+pub use texture::GpuTexture;