Fix format arg spans in diagnostics

[RAprogramm]

Summary

• enable the span-locations feature on proc-macro2 so join spans include location data
• fall back to the format argument value span when joining a named argument fails
• refresh trybuild stderr fixtures to capture the new span highlights

Testing

• cargo +nightly fmt --
• cargo +1.90.0 clippy -- -D warnings
• cargo +1.90.0 test --all
• cargo +1.90.0 doc --no-deps
• cargo deny check
• cargo audit

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https://chatgpt.com/codex/tasks/task_e_68da303f686c832b854c9408e537b614