Fix keyboard init: add timeout to controller flush loop

The PS/2 keyboard controller flush loop could hang infinitely on some
VMs (UTM) where the controller keeps reporting data. Add a 1024-byte
timeout to prevent the kernel from hanging during keyboard_init().

Also add debug prints showing flush progress for diagnostics.

src/keyboard.c

@@ -101,11 +101,21 @@ void keyboard_init(void) {
     kb_waiting_pid = 0;
     shift_pressed = 0;
 
-    /* Flush any pending data from the keyboard controller */
-    while (inb(0x64) & 0x01) {
+    offset_print("  KEYBOARD: flushing controller...\n");
+
+    /* Flush any pending data from the keyboard controller.
+     * Use a timeout to avoid hanging if the controller keeps reporting data
+     * (some emulators/VMs behave differently). */
+    int flush_count = 0;
+    while ((inb(0x64) & 0x01) && flush_count < 1024) {
         inb(KB_DATA_PORT);
+        flush_count++;
     }
 
+    offset_print("  KEYBOARD: flushed ");
+    print_hex((uint32_t)flush_count);
+    offset_print("  KEYBOARD: bytes, unmasking IRQ1...\n");
+
     /* Unmask IRQ1 (keyboard) in the PIC */
     pic_clear_mask(1);