[nicoburns]

Was auditing zeno for unsafe code. There is one instance:

unsafe { (v * 256.).to_int_unchecked() }

My LLM reckons that both:

• This usage is unsound (it is not validated and can be out of range, NaN, etc). See test case, which it has run under MIRI.
• Makes no difference to performance (it has run it's own benchmarks - not sure how representative they are)

This PR also:

• Removes some #[allow(clippy::uninit_assumed_init, invalid_value)] which are no longer needed (already the case on main - code that needed them was already removed).
• Adds #![forbid(unsafe_code)] (seeing as the crates now has no unsafe code)