p32: Add page table utilities and kernel memory mapping

[s-p-1]

Objective

This branch introduces the standalone x86 page directory utilities and maps the kernel's address space, strictly following xv6-public parity. It acts as the foundational step before migrating user processes off the GDT segmentation model.

Changes Made

• mmu.h: Added standard page directory and table constants (NPDENTRIES, PTE_P, PDXSHIFT, etc.).
• vm.c: Added walkpgdir(), mappages(), and setupkvm() to build the kernel page directory, plus kvmalloc() and switchkvm() to activate it.
• x86.h: Added lcr3() inline assembly.
• defs.h: Added respective prototypes.
• main.c: Inserted kvmalloc() immediately after kinit() to initialize the kernel page tables during boot.

Important Notes & Workarounds

• The PGSIZE Trap: The current p31 branch relies on PGSIZE being 1MB for the segmentation model. To prevent setupkvm() from wiping out 1MB of memory and crashing the OS, I explicitly forced #define PGSIZE 4096 locally at the top of vm.c. This allows the page tables to build correctly while maintaining OS-wide segmentation parity until the next branches.
• Dormant Paging: Paging is not explicitly turned on in CR0 yet, matching the incremental progression of xv6. CR3 is simply loaded and primed.