BIOS: implement get_key_timeout with INT 16h/01h polling + INT 15h/86h delay

Fixes autoboot countdown blocking forever because the default trait impl
calls blocking get_key(). Uses non-destructive key check (INT 16h AH=01h)
with EDX sentinel to detect key availability without EFLAGS access, and
INT 15h AH=86h for 10ms polling intervals.

src/os/bios/mod.rs

@@ -219,6 +219,54 @@ impl Os for OsBios {
         }
     }
 
+    fn get_key_timeout(&self, milliseconds: usize) -> OsKey {
+        let mut remaining = milliseconds;
+        loop {
+            let mut check = ThunkData::new();
+            check.eax = 0x0100;
+            check.edx = 0xFFFF_FFFF;
+            unsafe {
+                check.with(self.thunk16);
+            }
+
+            if check.edx != 0xFFFF_FFFF || check.eax != 0x0100 {
+                let mut data = ThunkData::new();
+                unsafe {
+                    data.with(self.thunk16);
+                }
+                return match (data.eax >> 8) as u8 {
+                    0x4B => OsKey::Left,
+                    0x4D => OsKey::Right,
+                    0x48 => OsKey::Up,
+                    0x50 => OsKey::Down,
+                    0x0E => OsKey::Backspace,
+                    0x53 => OsKey::Delete,
+                    0x1C => OsKey::Enter,
+                    _ => match data.eax as u8 {
+                        0 => OsKey::Other,
+                        b => OsKey::Char(b as char),
+                    },
+                };
+            }
+
+            if remaining == 0 {
+                return OsKey::Timeout;
+            }
+
+            let step = 10.min(remaining);
+
+            let mut wait = ThunkData::new();
+            wait.eax = 0x8600;
+            wait.ecx = 0;
+            wait.edx = (step * 1000) as u32;
+            unsafe {
+                wait.with(self.thunk15);
+            }
+
+            remaining -= step;
+        }
+    }
+
     fn clear_text(&self) {
         let mut vga = VGA.lock();
         vga.clear();