fix: VirtualBox compatibility — UART detection, alarm timing, usermode segments,...

fix: VirtualBox compatibility — UART detection, alarm timing, usermode segments, user-mode #GP

4 fixes for VirtualBox compatibility + 1 cosmetic:

1. UART hardware detection (fixes boot freeze with serial disabled)
   - hal_uart_init() now probes the scratch register before configuring
   - All UART operations (putc, drain_rx, poll_rx, try_getc) guarded
     behind uart_present flag — prevents infinite loop on floating bus
   - console_init() auto-enables VGA when no UART detected so boot
     messages are visible
   - Added hal_uart_is_present() API + stubs for ARM/MIPS/RISC-V

2. alarm/SIGALRM test: replace 20M-iteration busy-loop with nanosleep
   polling (50ms × 40 = 2s max wait). Fast VirtualBox CPUs completed
   the busy-loop before the 1-second alarm fired.

3. x86_enter_usermode: load DS/ES/FS/GS=0x23 before iret to ring 3.
   Without this, iret nulls segment registers (kernel DPL=0 < new CPL=3
   per Intel SDM §6.12.1). On QEMU this was masked by early context
   switches that fixed DS via x86_enter_usermode_regs, but VirtualBox
   with Hyper-V acceleration may expose the race window.

4. User-mode exception handling: deliver SIGSEGV for any ring-3
   exception (#GP, #UD, etc.) instead of kernel panic. Previously only
   #PF (14) had this handling. A user-mode #GP now kills the process
   cleanly instead of halting the entire system.

5. LAPIC timer ticks printed in decimal instead of hex.

89/89 smoke tests pass, cppcheck clean.