POSIX Tiers 1-3: signals, brk/sbrk, nanosleep, raw TTY, O_CLOEXEC, PMM refcount,...

POSIX Tiers 1-3: signals, brk/sbrk, nanosleep, raw TTY, O_CLOEXEC, PMM refcount, CoW fork, mmap/munmap, procfs, slab allocator, O(1) scheduler, PCI enumeration, VBE framebuffer

Tier 1:
- Signal characters (Ctrl+C/Z/\ -> SIGINT/SIGTSTP/SIGQUIT) in TTY/PTY
- brk/sbrk syscall with ELF loader heap break support
- nanosleep/clock_gettime syscalls (tick-based)
- Raw TTY mode (non-canonical) + ISIG flag + TIOCGWINSZ/TIOCSWINSZ

Tier 2:
- O_CLOEXEC/FD_CLOEXEC: per-FD flags, fcntl F_GETFD/F_SETFD, close on execve
- PMM ref-counting (uint16_t per-frame, atomic ops)
- Copy-on-Write fork: PTE bit 9 COW marker, page fault handler, vmm_as_clone_user_cow
- mmap/munmap: anonymous MAP_PRIVATE, per-process VMA tracker (32 slots)
- /proc filesystem: /proc/self/status, /proc/uptime, /proc/meminfo

Tier 3:
- Slab allocator: free-list-in-place, per-cache spinlock, auto-grow
- O(1) scheduler: 32-priority bitmap + active/expired runqueue swap
- PCI enumeration: bus/slot/func scan, vendor/device/class/BAR/IRQ
- VBE framebuffer: multiboot2 tag parsing, kernel VA mapping, pixel ops

All changes pass: make clean && make, cppcheck --enable=warning,performance,portability, and QEMU smoke test (10s stable boot, all init tests pass).