refactor(tinyinst): spring-clean TinyInstExecutor (#3796)

- Remove unnecessary unsafe (unwrap_unchecked -> if let Some)
- Remove #[expect(unused_assignments)] by restructuring control flow
- Replace .expect() with ? in Result-returning build()
- Use impl Into<String> / impl IntoIterator on builder APIs
- Drop redundant .to_string() on error messages
- Improve documentation with usage notes, safety invariants
- Use core::fmt::Result, Self return type, lifetime elision
- Move constants to top, reorder trait impls for readability
- Replace clone_from_slice with copy_from_slice
- Remove commented-out code

Author: erickcestari