Added a locked mmap arena for brk/sbrk, wired it into the libc build, updated host mmap handling, documented the issue in tasks.json, and introduced a covering Unity test.\n\nTests: make test

Architecture changes:
- Updated mem.md to describe userland brk/sbrk shim backed by mmap
- Updated per_process_vm.md to mark brk/sbrk as complete via #423
- Updated syscall_abi.md to clarify brk/sbrk are libc functions, not syscalls

Issue tracking:
- Marked #364 (stubbed functions) and #423 (brk/sbrk shim) as closed
- Updated stubbed functions status: 93% complete (13/14)
- Only gets() remains (deliberately not implementing - security vulnerability)

Dependency updates:
- Updated issues #55 and #56 to reflect brk/sbrk availability
- Removed incorrect references to issue #21 (PS/2 keyboard, not brk/sbrk)
- Updated dependency diagrams with corrected issue relationships

Diagrams:
- Refreshed libc and full dependency diagrams
- Regenerated PNG images to reflect current state