Problem

Variants currently support only variant-wide pre_built and wheel_server_url settings, which apply to all versions of a package. This creates conflicts when different collections need different build methods for the same package.

Example use case:

• Global TPU collection needs torch==2.9.0.dev20250730 as a prebuilt wheel from GitLab mirror
• torch-2.8.0 TPU collection needs torch==2.8.0 built from source
• Both use the same tpu-ubi9 variant

Currently impossible to configure this scenario without creating duplicate variants.

Solution

Add version-specific settings to allow per-version overrides within a single variant:

variants:
  tpu-ubi9:
    pre_built: false  # Default
    versions:
      "2.9.0.dev20250730":
        pre_built: true
        wheel_server_url: https://gitlab.com/.../simple
      "2.8.0":
        pre_built: false

Implementation

• Add VersionSpecificSettings model with pre_built, wheel_server_url, env, annotations
• Extend VariantInfo with versions mapping
• Add version-aware methods: is_pre_built(version), get_wheel_server_url(version)
• Update bootstrapper to use version-specific resolution
• Maintain full backward compatibility

Version-specific settings take precedence over variant defaults when present.