feat: donglora-cobs workspace crate with 31 spec-compliance tests

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).

crates/donglora-cobs/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "donglora-cobs"
+version = "0.1.0"
+edition = "2021"
+license = "MIT"
+description = "COBS encoder/decoder — no_std, zero-alloc, spec-compliant"
+
+[features]
+default = []