feat: implement 32-bit protected mode transition

[FelineFantasy]

Summary

Implement 32‑bit protected mode transition for CatOS.

Currently, CatOS runs in 16‑bit real mode, which limits memory access to 1 MB. This issue adds support for 32‑bit protected mode, unlocking more memory and modern CPU features.

Open questions

• Should we keep a fallback to real mode for debugging?
• Which GDT layout (flat or segmented) fits best for CatOS?
• Do we need to handle interrupts (IDT) in this PR or leave it for a follow‑up?

Phase 0 (this issue)

• Define GDT with null, code, and data descriptors
• Load GDT using lgdt
• Set PE (Protection Enable) bit in cr0
• Perform a far jump to a 32‑bit code segment
• Test boot in QEMU and log the new state

Status: exploring - not yet scheduled for a specific release.