Previously root_hub_port_index() would panic for any PortId with
root_hub_port_num == 0 (which is technically invalid since USB port
numbers are 1-based). Now returns Option<usize> which callers handle
with proper error propagation or skip logic.
Updated 7 call sites across mod.rs, irq_reactor.rs, device_enumerator.rs,
and scheme.rs to handle the new Option return type. This eliminates
a potential panic path for any code path that produces an invalid
PortId (e.g., from a malformed /scheme/usb/ URI).
Cross-referenced with Linux 7.1 drivers/usb/core/hub.c usb_hub_find_child()
which validates port numbers with bounds checks.
Resolves IMPROVEMENT-PLAN §2.5: 'Fix PortId::root_hub_port_index() panic'.
Red Bear OS
72ba66efb9