Smart pointer and resource ownership patterns for Rust heap allocation and reference counting.

• Guides ownership decisions through a three-step model: single vs. shared ownership, single-threaded vs. multi-threaded context, and presence of reference cycles
• Covers six core types (Box, Rc, Arc, Weak, Cell, RefCell) with a decision flowchart and quick reference table for choosing the right pattern
• Includes common errors and anti-patterns with fixes, such as using Weak to break cycles, avoiding unnecessary Arc overhead, and preventing RefCell panics
• Traces design questions upward to concurrency and mutability concerns, and downward to concrete implementation choices