Extract COBS encoder/decoder into a separate no_std workspace crate
(crates/donglora-cobs) with comprehensive tests against the Cheshire &
Baker 1999 IEEE/ACM specification and Wikipedia canonical vectors.
Tests cover: empty input, single/multiple zeros, block boundaries at
254/255 bytes, alternating patterns, the Ping regression case (tag 0x00),
round-trip verification, and error cases (truncation, unexpected zeros).
Replace inline cobs_decode() in usb/task.rs with the tested crate.
Replace external cobs dependency with workspace crate.
Fix encoder bug at 254-byte block boundary (spurious trailing code byte).
