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).

Cargo.lock

@@ -119,15 +119,6 @@ dependencies = [
  "num-traits",
 ]
 
-[[package]]
-name = "cobs"
-version = "0.3.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0fa961b519f0b462e3a3b4a34b64d119eeaca1d59af726fe450bbba07a9fc0a1"
-dependencies = [
- "thiserror",
-]
-
 [[package]]
 name = "cortex-m"
 version = "0.7.7"
@@ -336,11 +327,11 @@ name = "donglora"
 version = "0.1.0"
 dependencies = [
  "cfg-if",
- "cobs",
  "cortex-m-rt",
  "critical-section",
  "defmt 0.3.100",
  "display-interface",
+ "donglora-cobs",
  "embassy-embedded-hal",
  "embassy-executor",
  "embassy-futures",
@@ -364,6 +355,10 @@ dependencies = [
  "static_cell",
 ]
 
+[[package]]
+name = "donglora-cobs"
+version = "0.1.0"
+
 [[package]]
 name = "embassy-embedded-hal"
 version = "0.2.0"