chore(ci): align toolchain to stable (fix cargo-fmt in CI)

[kfastov]

Why:

The Rust version is misaligned between CI and rust-toolchain.toml:

• CI installs stable rustfmt/clippy;
• rust-toolchain.toml requires nightly;

This causes cargo fmt to fail during CI.

Proposition:

• We can safely change pinned toolchain version to "stable" because the repo doesn't use any nightly features yet

Changes:

• Set rust-toolchain.toml to channel = "stable".
• Fix clippy warnings to ensure CI passes

Result: build/fmt/clippy/tests run on stable; CI passes.

[kfastov]

@lmlmt What do you think?

[lmlmt]

@lmlmt What do you think?

Sounds good, thanks!