Add selectable CUDA library modes

[cjm-dd]

Summary

Add a repository-wide @cuda//:library_mode build setting so consumers can switch CUDA linkage without changing dependency labels. The flag accepts shared, static, or system and defaults to the existing shared behavior.

For each complete foo / foo_static / foo_system family, the generated proxy now keeps the explicit suffixed targets stable and makes the unsuffixed foo target select the requested variant. Targets that do not expose a complete family are unchanged. cuDART also gains the semantic cudart_static alias needed to participate consistently.

The proxy already selects concrete repositories by CUDA version. The generator now composes that dimension with linkage mode through private per-mode version aliases, then exposes a single public mode-selecting alias. This preserves existing labels and defaults while avoiding nested configurable attributes.

Validation

• bazel --nohome_rc build //...
• Analysis-only e2e builds of the unsuffixed cuBLAS consumer for shared, static, and system across CUDA 12.8.1, 12.9.1, 13.0.2, 13.1.1, 13.2.1, and 13.3.1
• Real downstream Bzlmod link builds against unsuffixed cuBLAS and cuDART targets in all three modes using a local path override

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