Function route_legacy_irq 

pub fn route_legacy_irq(
    irq: u8,
    lapic_id: u32,
    vector: u8,
    overrides: &[Option<InterruptSourceOverride>],
)

Route a legacy ISA IRQ, applying MADT interrupt source overrides.

This handles the common case of IRQ0→GSI2 remapping on QEMU q35.