feat: add benchmarks, justfile, optimize throughput, harden quality gates

Benchmarking & comparison:
- Add criterion benchmarks (ucobs vs cobs 0.3 vs corncobs 0.1)
- Add Python cobs benchmark script
- Add performance comparison section to README with full results

Performance:
- Rewrite encode to scan-and-bulk-copy via copy_from_slice
- Rewrite decode to bulk-copy via copy_from_slice
- ~2-4x throughput improvement; now matches corncobs on most workloads

Quality hardening:
- Add #![forbid(unsafe_code)], #![warn(missing_docs)], #[must_use]
- Add [lints.rust] unsafe_code = "forbid" in Cargo.toml
- Add rust-version (MSRV) = "1.56"
- Add docs.rs metadata
- 106 tests: 80 unit/corpus + 25 property tests (25k cases each = 625k total)
- External test vectors from: Wikipedia, Cheshire & Baker paper, nanocobs,
  cobs-c, cobs2-rs, Jacques Fortier, Python cobs package
- Add fuzz_small_dest fuzz target (encode/decode with undersized buffers)
- Improve doc examples to show sentinel append/strip convention

Infrastructure:
- Add justfile with test (nextest), check, bench, fuzz, miri, fmt targets
- cargo-nextest for parallel test execution
- Update .gitignore for Python cache files

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

fuzz/Cargo.toml

@@ -24,3 +24,9 @@ name = "fuzz_roundtrip"
 path = "fuzz_targets/fuzz_roundtrip.rs"
 test = false
 doc = false
+
+[[bin]]
+name = "fuzz_small_dest"
+path = "fuzz_targets/fuzz_small_dest.rs"
+test = false
+doc = false

fuzz/fuzz_targets/fuzz_small_dest.rs

@@ -0,0 +1,36 @@
+//! Fuzz target: encode and decode with undersized destination buffers.
+//! Must never panic — should return None when dest is too small.
+#![no_main]
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|data: &[u8]| {
+    if data.is_empty() {
+        return;
+    }
+
+    // Split fuzz input: first byte selects dest size, rest is payload.
+    let dest_size = data[0] as usize;
+    let payload = &data[1..];
+
+    // Encode with a potentially too-small buffer.
+    let mut enc_buf = vec![0u8; dest_size];
+    let enc_result = ucobs::encode(payload, &mut enc_buf);
+
+    if let Some(enc_len) = enc_result {
+        let encoded = &enc_buf[..enc_len];
+
+        // Encoded output must never contain 0x00.
+        assert!(!encoded.contains(&0x00), "encoded contains sentinel");
+
+        // Decode with a potentially too-small buffer.
+        let mut dec_buf = vec![0u8; dest_size];
+        let dec_result = ucobs::decode(encoded, &mut dec_buf);
+
+        if let Some(dec_len) = dec_result {
+            assert_eq!(
+                &dec_buf[..dec_len], payload,
+                "round-trip mismatch with small buffers"
+            );
+        }
+    }
+});