Update Rust toolchain to 1.91.1

Update from 1.82.0 to 1.91.1 to match current stable and ensure
CI uses the correct version for clippy lints.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: FunKite

rust-toolchain.toml

@@ -1,5 +1,5 @@
 [toolchain]
-channel = "1.82.0"
+channel = "1.91.1"
 components = ["rustfmt", "clippy", "rust-src"]
 targets = ["x86_64-unknown-linux-gnu", "aarch64-unknown-linux-gnu", "x86_64-apple-darwin", "aarch64-apple-darwin", "x86_64-pc-windows-msvc"]
 profile = "default"