chore: audit cleanup, CI, cargo-deny, encode fuzz target (0.3.4)

Simplify encoder exit branch (−41 B code size), add GitHub Actions CI
with MSRV matrix, cargo-deny with permissive-only license policy,
encode-only fuzz target, and feature coverage across test gates.

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

fuzz/Cargo.toml

@@ -30,3 +30,9 @@ name = "fuzz_small_dest"
 path = "fuzz_targets/fuzz_small_dest.rs"
 test = false
 doc = false
+
+[[bin]]
+name = "fuzz_encode"
+path = "fuzz_targets/fuzz_encode.rs"
+test = false
+doc = false

fuzz/fuzz_targets/fuzz_encode.rs

@@ -0,0 +1,33 @@
+//! Fuzz target: encode arbitrary input into buffers of varying sizes.
+//! Exercises encode error paths (buffer too small) and validates output properties.
+#![no_main]
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|data: &[u8]| {
+    if data.len() > 1024 {
+        return;
+    }
+
+    let max = ucobs::max_encoded_len(data.len());
+
+    // Encode into a correctly-sized buffer — must always succeed.
+    let mut buf = vec![0u8; max];
+    let n = ucobs::encode(data, &mut buf)
+        .expect("encode failed with max_encoded_len buffer");
+    let encoded = &buf[..n];
+
+    // Output must never contain 0x00.
+    assert!(!encoded.contains(&0x00), "encoded output contains 0x00");
+
+    // Output length must be bounded by max_encoded_len.
+    assert!(n <= max, "encoded length exceeds max_encoded_len");
+
+    // Output must be at least 1 byte (even empty input encodes to [0x01]).
+    assert!(n >= 1, "encoded length is zero");
+
+    // Encode into every possible undersized buffer — must return None, never panic.
+    for size in 0..n {
+        let mut small = vec![0u8; size];
+        assert_eq!(ucobs::encode(data, &mut small), None);
+    }
+});