Enable clippy linting in repo

[lionel-]

https://github.com/rust-lang/rust-clippy

We use it in air: https://github.com/posit-dev/air/blob/main/clippy.toml

And it's used in most major Rust projects:

• https://github.com/biomejs/biome/blob/main/clippy.toml
• https://github.com/rust-lang/rust-analyzer/blob/master/clippy.toml
• https://github.com/astral-sh/ruff/blob/main/clippy.toml
• https://github.com/zed-industries/zed/blob/main/clippy.toml

In addition to linting towards conventional style conventions, it also makes it possible to disallow use of certain methods, functions, or types, which could be handy in the future (see clippy.toml files above for examples of this). We could disallow .unwrap() and .expect() in favour of log_err or explicit panic for instance. This is nice especially with agentic workflows since style directives might be ignored by agents but they can't ignore lint rules.

[lionel-]

One UX downside to be aware of: After each edits, clippy runs longer checks on the repo, which locks other gestures requiring compilation (running tests etc).