[codex] support variable packet lengths for isochronous transfers

[ZR233]

What changed

• carry explicit per-packet lengths through TransferKind::Isochronous instead of only storing a packet count
• add submit_with_packet_lengths helpers for isochronous endpoints while keeping the even-split fallback API
• update the xHCI and userspace backends to enqueue/fill ISO packets using the provided packet lengths
• expose DeviceInfo::id() for callers that need a stable device identifier

Why

Some isochronous workloads need uneven packet sizing, but the previous transfer model only kept num_pkgs and then recomputed even slices in each backend. That could not represent variable packet layouts correctly.

Impact

• callers can now submit isochronous transfers with exact packet boundaries
• backend packet descriptors and TRB layout stay aligned with the caller-provided packet sizes
• existing callers that only know num_packets still work through the even-split helper

Validation

• cargo fmt --check
• cargo check -p crab-usb --lib
• cargo check -p crab-uvc --lib
• attempted cargo test -p crab-usb -p crab-uvc but it still fails in pre-existing crab-uvc tests/examples (VideoFormat::* variant resolution and missing USBHost::new_libusb), unrelated to this diff