fix(rust-patterns-book): make Lifetime Branding example compile

[rodrirejala]

Fix #102
The Lifetime Branding example in ch04 failed to compile because alloc took
&'a mut self and returned ArenaHandle<'a>, keeping the mutable borrow alive.
Calling arena1.get(handle1) then tried to immutably borrow arena1 while it
was still mutably borrowed → error[E0502].
Solution: Use RefCell<Vec<String>> for interior mutability so alloc takes
&self instead of &mut self. The handle's lifetime is tied to an immutable
borrow, allowing get to coexist without borrow conflicts.
Changes:

• Replace Vec<String> with RefCell<Vec<String>>
• alloc: &'a mut self → &self, use borrow_mut() for mutation
• get: &'a self, ArenaHandle<'a> → &'a str → retains 'a constraint, returns String via clone
• main: let mut arena1 → let arena1 (no longer needs mut)
  get returns a cloned String instead of &str — acceptable simplification for
  a teaching example focused on PhantomData and lifetime branding rather than
  zero-copy arena design.