feat: implement kconsole — kernel emergency console (like HelenOS kconsole)

feat: implement kconsole — kernel emergency console (like HelenOS kconsole)

When VFS mount or init fails, AdrOS now enters a minimal kernel-mode
emergency console instead of the full shell. This is similar to
HelenOS's kconsole — it runs entirely in kernel mode and provides
basic diagnostic commands.

Design:
- kconsole_enter() runs a blocking read loop using kgetc()/kprintf()
- Commands: help, dmesg, reboot, halt
- Activated from kernel_main() when init_start() returns < 0
- Fully architecture-independent (uses only generic console API)

The normal shell remains available for userspace-initiated sessions.

Passes: make, cppcheck, QEMU smoke test.