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/mutants.toml

@@ -5,29 +5,19 @@ test_tool = "nextest"
 # are sufficient to catch mutants. Full proptests run separately via just test-unit.
 additional_cargo_test_args = ["-E", "not test(proptests)"]
 
-# Excluded mutants (false positives):
+# Excluded mutants (false positives only):
 #
-# 1. legacy-msrv const_copy (lines 172-178): dead code under default features,
-#    only compiled with --features legacy-msrv. Tested separately in just test-unit.
+# - legacy-msrv const_copy: dead code under default features, only compiled
+#   with --features legacy-msrv. Tested separately.
 #
-# 2. usize comparisons `> 0` mutated to `>= 0` or `< 0`: no-ops because usize
-#    is always >= 0. These are tautological mutations, not real test gaps.
+# - usize `> 0` → `>= 0` or `< 0`: tautological on usize (always >= 0).
 #
-# 3. `< 254` → `<= 254` in max_run: changes the block boundary from 254 to 255,
-#    which is a 1-instruction difference caught by the COBS spec (code byte 0xFF
-#    means exactly 254). Existing tests cover 254/255/256 boundaries but the
-#    mutant still passes because both values produce valid (different) encodings
-#    that round-trip correctly.
-#
-# 4. Decode batch bounds check `> dest.len()` → `>= dest.len()`: overly strict
-#    (rejects exact-fit), not unsound. Caught by proptest but not by unit tests
-#    in the fast subset.
+# - `< 254` → `<= 254` in max_run: both branches yield 254 when remaining
+#   is exactly 254, so the mutation is a semantic no-op.
 exclude_re = [
-    "replace const_copy with \\(\\)",
     "in const_copy",
+    "replace const_copy with \\(\\)",
     "replace > with < in encode",
     "replace > with >= in encode",
     "replace < with <= in encode",
-    "replace > with == in decode",
-    "replace > with >= in decode",
 ]