feat: implement Phase 1 foundation - triangle rendering

- voltex_math: Vec3 with arithmetic ops, dot, cross, length, normalize
- voltex_platform: VoltexWindow (winit wrapper), InputState (keyboard/mouse),
  GameTimer (fixed timestep + variable render loop)
- voltex_renderer: GpuContext (wgpu init), Vertex + buffer layout,
  WGSL shader, render pipeline
- triangle example: colored triangle with ESC to exit

All 13 tests passing. Window renders RGB triangle on dark background.

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

crates/voltex_renderer/src/lib.rs

@@ -1,2 +1,5 @@
-// Voltex Renderer - Phase 1
-// Modules will be added in Task 4
+pub mod gpu;
+pub mod pipeline;
+pub mod vertex;
+
+pub use gpu::GpuContext;