[cellrishi-code]

Summary

Adds the x86_64 divide-error (#DE / vector 0) exception path to Rustalis.

Changes

• Register a dedicated divide-error handler in the static IDT.
• Log the faulting RIP and code segment before entering the kernel panic path.
• Add unit coverage verifying the #DE IDT entry is populated.
• Add an opt-in divide-error-self-test feature using Rust inline assembly to intentionally trigger integer division by zero.
• Compile both exception self-test features in CI.
• Document the new exception boundary and self-test.

Closes #32

Dependency

This is a stacked PR on #33 (feat/exception-handlers). Once #33 is merged, this PR can be retargeted to main and merged independently.

The normal boot path does not intentionally trigger a divide error.