Warn on always false contract predicates

[robjtede]

Summary

• detect literal false contract predicates during attribute parsing
• rewrite them through a deprecated const so stable Rust emits a compile-time warning
• add an MSRV-gated trybuild fixture that asserts the warning via #![deny(deprecated)]

Closes #26.

Validation

• cargo +1.65.0 test --test ui
• just clippy
• just test

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