Rename framework type from cargo to nextest in TOML configs

[jsk11235]

Summary

• Rename type = "cargo" to type = "nextest" in the three offload-cargo-*.toml config files
• Add #[serde(rename = "nextest")] to the Cargo variant in FrameworkConfig so TOML deserialization accepts the new name
• Update the corresponding test TOML literal in schema.rs

Test plan

• cargo fmt --check passes
• cargo clippy passes (no warnings)
• cargo nextest run passes (128/128 tests)

🤖 Generated with Claude Code

[github-actions]

Vet found 1 issue.

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README.md:204

[documentation_implementation_mismatch] (severity 3/5) (confidence 0.90)

The README.md references to the framework type cargo are inconsistent with the rename to nextest. The CLI Reference for offload init lists cargo as a valid --framework TYPE option, and the Configuration Reference / Example Configurations sections show type = "cargo". All of these should be updated to nextest to match the new config schema.

[github-actions]

Vet found 1 issue.

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README.md:376

[documentation_implementation_mismatch] (severity 3/5) (confidence 0.95)

The README.md contains example configuration snippets that still use type = "cargo" instead of the new type = "nextest". For example, the 'Local Cargo Tests' and 'Cargo Tests on Modal' examples show type = "cargo" in the [framework] section. These should be updated to type = "nextest" to match the rename applied to the offload-cargo-*.toml config files.

[DanverImbue]

LGTM