Bump rust-version to 1.84 to match faer 0.24 MSRV

faer 0.24 requires Rust 1.84, which is higher than PyO3 0.28's 1.83
floor. Declare the actual minimum so source builds fail early.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: igerber

rust/Cargo.toml

@@ -2,7 +2,7 @@
 name = "diff_diff_rust"
 version = "3.0.0"
 edition = "2021"
-rust-version = "1.83"
+rust-version = "1.84"
 description = "Rust backend for diff-diff DiD library"
 license = "MIT"