feat(models): complete DeepSeek-VL2 VLM port

[inureyes]

Summary

Completes the DeepSeek-VL2 (deepseek_vl_v2) port, building on the WIP SigLIP tower and the deepseek_v2 embeds-forward helpers.

• Vision tower (src/vision/encoders/deepseek_vl2.rs): SigLIP-style no-CLS ViT (patch conv folded to a linear, learned pos_embed, fused-qkv full attention, GELU-tanh MLP, trailing LayerNorm). Pixel activations are cast to the tower weight dtype so a bf16 pixel batch meets the f16 tower.
• Projector (src/vision/connectors/deepseek_vl2.rs): downsample_mlp_gelu (2x2 space-to-depth with channel-outermost feature order, then Linear, GELU, Linear).
• Processor (src/vision/processors/deepseek_vl2.rs): candidate-resolution best-fit tiling (max effective area, min-wasted tie-break), mean-color padding, a global thumbnail plus local tiles.
• Wrapper (src/vision/deepseek_vl2.rs): the 2D tile mosaic (image_newline columns, view_separator, order [global, view_separator, local]) scattered into the <image> positions, over the deepseek_v2 MoE decoder reused unchanged.
• Loader / wiring: vlm_deepseek_vl2.rs, detection, runtime dispatch, prompt expansion, model registration, and docs.

The text backbone reuses deepseek_v2.rs with no changes: config.json omits the MLA head dims, so the ModelArgs serde defaults match DeepSeek-V2, and the checkpoint ships pre-fused switch_mlp experts. The <image> id is 100003 (the tokenizer's <image>; the config carries no image_token_index).

Validation

Real checkpoint mlx-community/deepseek-vl2-small-4bit:

• Text-only: "What is the capital of France?" produced "The capital of France is Paris."
• Solid image: "What is the main color?" produced "The main color of this image is orange."
• Shapes image (448px, 2x2 grid, 1023 image tokens): produced "three geometric shapes: a red circle, a blue square, and a green triangle."

9 co-located unit tests and 4 real-model parity tests (tests/deepseek_vl_v2_parity.rs) pass. Release build (metal,accelerate), cargo fmt, and cargo clippy --all-targets -- -D warnings are clean.

Drive-by

Clears two pre-existing clippy findings surfaced by a newer local toolchain: a needless return in build.rs (the CUDA recipe after it is cfg(not(macos)), so nothing runs after on macOS) and a runtime assertion on constants in switch_layers.rs (moved into a const block).

Closes #533