refactor: introduce Transport abstraction and sysfs backend

[hagz0r]

This PR refactors the cd321x device logic to be transport-agnostic by introducing
a Transport trait, and moves the existing sysfs-based implementation into a
dedicated backend.

Motivation:

• decouple protocol/device logic from the concrete I/O mechanism
• make it possible to add alternative transports in the future
  (e.g. mock/testing, userspace bridges, other host OSes)
• reduce cd321x-specific filesystem handling in higher-level code

Changes

• introduce a Transport trait defining read/write semantics
• rework cd321x::Device to operate on a generic Transport
• move current sysfs implementation into transport::sysfs
• no functional behavior changes intended

This is a pure refactor; behavior and command formats are unchanged.

Tested by building and running the tool against the existing sysfs backend.