test: mutation testing, block boundary tests, exact-fit decode tests (0.3.2)

Added cargo-mutants with 88 mutants tested, 59 caught, 0 missed. Added
targeted tests for 254-byte block boundary encoding (interop-verified)
and exact-fit/too-small decode buffer sizing for the batch zero path.
111 tests total (was 106).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "ucobs"
-version = "0.3.1"
+version = "0.3.2"
 edition = "2021"
 rust-version = "1.93"
 license = "MIT"