Consider changing `check()` to return `Result<bool, E>`

[moodmosaic]

Right now Command::check() returns a bool. This is simple, but makes it hard to debug when something fallible happens inside a check (e.g. lock acquisition, DB access).

Normally check() should avoid side effects — but we can't enforce that at the type level in Rust. We're not in Haskell. Implementers may do I/O or locking, and panics or silent failures can short-circuit the test run with no explanation.

We're considering switching to:

fn check(&self, state: &S) -> Result<bool, E>;

This would let the runner surface useful diagnostics without panicking or swallowing errors. We could start with E = &'static str or Box<dyn std::error::Error>.

Not settled yet — opening this for discussion. Credit to @cylewitruk for raising the point.