usb: a device has as many interfaces as it declares

max_interfaces was 4. A composite device — a headset, a webcam with audio, a
dock, a multifunction printer — routinely has more, and the fifth did not
merely go missing. parseConfiguration's cap branch had no `else`, so when the
count was reached `current` kept pointing at interface 3 and the fifth
interface's endpoint descriptors were appended to interface 3's array. A
class driver bound to interface 3 could then be handed an endpoint belonging
to something else entirely, and subscribe or bulk-transfer on it. The
alternate-setting arm one line above cleared `current` correctly, which is
what the cap branch should have done.

Interfaces are now counted from the block in a first pass and allocated to
exactly that number, so the ceiling is bNumInterfaces' u8 — the USB
specification's. The missing `else` is added too, though after this the bug
is unreachable by construction: interface_count cannot reach interfaces.len
mid-parse when the list was sized from the same walk.

max_configured_endpoints was max_interfaces * max_endpoints_per_interface =
16, a derived guess that moved whenever either input moved. It is now 31,
which is the xHCI specification's own limit: a Device Context holds a slot
context plus at most 31 endpoint contexts, because the Context Entries field
addressing them is 5 bits.

max_endpoints_per_interface stays at 4 with its reason recorded — the
usb-transfer wire protocol reports exactly max_reported_endpoints (4) per
interface, so widening it alone would change nothing a class driver sees.
Lifting it is a protocol change.

No direct test, and that is written down as open question 5 rather than
glossed. The parser is pure and wants a host unit test, but
usb-xhci-library.zig imports memory, mmio and time so it cannot be a
standalone test root, and QEMU offers nothing that reaches the path — the
largest device available is usb-audio,multi=on at 2 interfaces and 211
bytes. The alternate-setting path that shares the same `current = null`
logic is exercised by that device.

Suite 116/116.

tools/bounds-allowlist.txt

@@ -82,8 +82,6 @@ system/drivers/usb-xhci-bus/usb-xhci-library.zig:data
 system/drivers/usb-xhci-bus/usb-xhci-library.zig:descriptor
 system/drivers/usb-xhci-bus/usb-xhci-library.zig:head
 system/drivers/usb-xhci-bus/usb-xhci-library.zig:header
-system/drivers/usb-xhci-bus/usb-xhci-library.zig:max_endpoints_per_interface
-system/drivers/usb-xhci-bus/usb-xhci-library.zig:max_interfaces
 system/drivers/usb-xhci-bus/usb-xhci-library.zig:max_subscriptions
 system/drivers/usb-xhci-bus/usb-xhci-library.zig:port_changes
 system/drivers/usb-xhci-bus/usb-xhci-library.zig:raw