Add trybuild dev-dependency and compile-time UI harnesses for cfg(kani) gating

[coderabbitai]

Summary

The repository has no trybuild dev-dependency and no tests/ui directory. PR #336 introduces #[cfg(kani)] conditional compilation paths and an unexpected_cfgs lint entry; there are no compile-pass or compile-fail UI tests to exercise these at compile time.

Required work

1. Add trybuild as a dev-dependency in Cargo.toml.
2. Create a tests/ui/ directory with at least:
   • A compile-pass snippet confirming #[cfg(kani)] code compiles cleanly under normal cargo build.
   • A compile-fail snippet confirming that a bare cfg(unknown_feature) triggers the unexpected_cfgs warning.

References

• PR: Kani harnesses for manifest-to-IR safety checks (4.2.1) #336
• Flagged by: @leynos

[coderabbitai]

🔗 Related PRs

#305 - Add Kani tooling and local smoke targets (4.1.1) [merged]
#308 - Add Kani smoke CI job (4.1.2) [merged]
#309 - Expand top-level action flow control (3.14.2) [merged]
#333 - Document cycle detector state ownership [merged]
leynos/cuprum#110 - Stabilise invalid pymodule compile-fail baseline [merged]

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