Metal render pipeline: 2D shaders and vertex format #290

Implements issue 3mi435c6kgh2i

Summary #

• MSL vertex shader: transforms pixel coordinates to clip space via viewport uniform
• MSL fragment shader: dual-mode (solid color fill + textured with color tint)
• Vertex format: position, color, tex_coord, use_texture (36 bytes, matches MSL packed layout)
• MTLRenderPipelineState with alpha blending (source * sourceAlpha + dest * (1 - sourceAlpha))
• MetalRenderer struct: batched quad rendering with vertex buffer building and draw call encoding
• Device methods: shader compilation, pipeline/buffer/texture/sampler creation
• RenderCommandEncoder methods: pipeline state, vertex buffer, uniforms, texture, sampler, draw
• 1×1 white dummy texture for solid-color rendering without unbound resources
• 23 tests covering struct layout, quad triangulation, shader compilation, pipeline creation, and MetalRenderer