p36: Standardize PGSIZE to 4KB and integrate dynamic heap management

[2023cs50578]

Objective

This branch pays off the technical debt from the high-half migration by standardizing the OS to a 4KB page resolution. It removes the intermediate boot hacks required by the legacy 1MB page size and fully wires the user-space heap allocation system (sbrk) to the hardware paging utilities.

Core Changes

• mmu.h & exec.c: Redefined the global PGSIZE from the legacy segmentation size (1MB) to the standard 4096 bytes. Removed the local #undef PGSIZE override in exec.c now that the global definition is correct.
• proc.c (growproc): Rewrote the heap management logic. Instead of blindly increasing p->sz, growproc now explicitly requests physical page allocation/deallocation via allocuvm and deallocuvm. Added a switchuvm call to flush the hardware TLB when the page directory changes.

Boot-Path Reversions

• main.c: With kalloc now dispensing 4KB pages, the setupkvm memory footprint is drastically reduced. Reverted the 128MB entrypgdir mapping expansion back to the standard 2-entry xv6-public configuration.
• main.c (kinit1): Reverted the kinit1 initialization boundary back to 4MB, as the allocator now has plenty of granular pages to bootstrap the full kpgdir.