feat: increase timer frequency to 100Hz (like Linux)

feat: increase timer frequency to 100Hz (like Linux)

- Central TIMER_HZ=100 and TIMER_MS_PER_TICK=10 constants in timer.h
- Replace all hardcoded 50Hz / 20ms-per-tick assumptions:
  syscall.c (nanosleep, clock_gettime), procfs.c (/proc/uptime),
  net_ping.c (sleep/time calculations), sys_arch.c (sys_now),
  sync.c (ksem_wait_timeout ms-to-ticks conversion)
- Use timer_init(TIMER_HZ) instead of hardcoded 50
- BSP-only timer tick via lapic_get_id() — APs return early to
  eliminate sched_lock/vga_lock contention at higher tick rates
- vga_flush() skips spinlock + cursor update when nothing dirty
- Preempt every 2nd tick (effective 50Hz preemption) to avoid
  excessive CR3 reloads in emulated environments (QEMU TLB flush
  overhead). Sleep/timing resolution remains at full 100Hz via
  process_wake_check on every tick.

Tests: 20/20 smoke (11s), 16/16 battery, cppcheck clean