Rust mutability design: choosing between &mut, interior mutability, and thread-safe patterns.

• Guides decision-making for E0596, E0499, E0502 errors by asking whether mutation is necessary and who controls it, rather than reflexively adding mut
• Covers single-thread patterns (Cell, RefCell) and multi-thread patterns (Mutex, RwLock, Atomic types) with clear selection criteria
• Includes borrow rule reference, anti-patterns (RefCell overuse, Mutex in hot loops), and decision tables for interior mutability scenarios
• Traces upward to data structure design (m09-domain) and concurrency concerns (m07-concurrency) when borrow conflicts persist