Real Time Clock

Implement `days_in_month()` and `is_leap_year()`. Why it matters: Leap-year-aware month math is the base layer for every RTC conversion added later in the phase. Focus on `days_in_month` and `is_leap_year` in `kernel-core/src/time.rs` while you trace the snippet.

/// Returns the number of days in the given month (1-indexed) for the given year.
///
/// Panics if `month` is 0 or greater than 12.
pub fn days_in_month(year: u32, month: u32) -> u32 {
    match month {
        1 => 31,
        2 => {
            if is_leap_year(year) {
                29
            } else {
                28
            }
        }
        3 => 31,
        4 => 30,
        5 => 31,
        6 => 30,
        7 => 31,
        8 => 31,
        9 => 30,
        10 => 31,
        11 => 30,
        12 => 31,
        _ => panic!("invalid month"),

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Implement CMOS register read function. Why it matters: A minimal raw CMOS accessor is the hardware boundary that the rest of the RTC driver can safely build on. Focus on `cmos_read` in `kernel/src/rtc.rs` while you trace the snippet.

/// Read a single CMOS register.
///
/// Port 0x70 is the address/NMI-disable port (bit 7 disables NMI).
/// Port 0x71 is the data port.
///
/// # Safety
///
/// Performs raw x86 port I/O. Must only be called in ring 0.
unsafe fn cmos_read(register: u8) -> u8 {
    let mut addr_port = Port::<u8>::new(0x70);
    let mut data_port = Port::<u8>::new(0x71);
    unsafe {
        addr_port.write(register | 0x80); // bit 7 = disable NMI during read
        let value = data_port.read();
        // Re-enable NMI by writing the register index with bit 7 clear.
        addr_port.write(register & 0x7F);
        value
    }
}

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Wire RTC module into kernel build. Why it matters: Boot-time RTC initialization is what replaces the earlier monotonic-only startup state with a real wall-clock epoch. Focus on `rtc::init_rtc` in `kernel/src/main.rs` while you trace the snippet.

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