feat: implement shared memory IPC (shmget/shmat/shmdt/shmctl)

feat: implement shared memory IPC (shmget/shmat/shmdt/shmctl)

Add System V-style shared memory IPC subsystem:
- src/kernel/shm.c: kernel-side segment manager with up to 32
  segments, each up to 16 pages (64KB). Physical pages allocated
  via PMM, mapped into user address space via VMM.
- include/shm.h: API + constants (IPC_CREAT, IPC_EXCL, IPC_RMID,
  IPC_PRIVATE)
- Syscalls 46-49: SHMGET, SHMAT, SHMDT, SHMCTL wired in syscall.c
- shm_init() called from kernel_main after kheap_init
- Deferred destruction: IPC_RMID with nattch>0 defers free until
  last detach

Also fixes:
- tty.c: add utils.h include for memset (cross-compiler strictness)
- usermode.c: fix ebp clobber error with cross-compiler by using
  ESI as scratch register instead

Passes: make, cppcheck, QEMU smoke test.