fix(unsafe): use unchecked_add in may_overflow to accurately model UB

[Aditya-9-6]

Summary

Updates the may_overflow example in the Unsafe Deep Dive chapter to use i32::unchecked_add instead of standard +.

Background

In standard Rust, integer overflow (a + i32::MAX) is not undefined behavior: it panics with overflow checks enabled (debug mode) and performs two's-complement wrapping in release mode.

Using unchecked_add:

1. Genuinely justifies the unsafe function signature and block.
2. In debug builds, it triggers the standard library's precondition panic:

   unsafe precondition(s) violated: i32::unchecked_add cannot overflow
   

3. In release builds, it produces actual Undefined Behavior (UB) as the slide intends to teach.

The speaker notes have been updated accordingly to clarify the distinction between safe Rust arithmetic (which wraps in release mode) and unchecked operations (which produce UB).

Fixes #3122
Relates #3021

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.