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>

bench_python.py

@@ -0,0 +1,67 @@
+#!/usr/bin/env python3
+"""Benchmark Python cobs encode/decode vs ucobs (reference timing only).
+
+Requires: pip install cobs
+"""
+
+import time
+
+
+def payload_zeros(n: int) -> bytes:
+    return b"\x00" * n
+
+
+def payload_nonzero(n: int) -> bytes:
+    return bytes((i % 255 + 1) for i in range(n))
+
+
+def payload_mixed(n: int) -> bytes:
+    return bytes(i % 256 for i in range(n))
+
+
+def bench(func, data, iterations=10_000):
+    start = time.perf_counter()
+    for _ in range(iterations):
+        func(data)
+    elapsed = time.perf_counter() - start
+    return elapsed
+
+
+def main():
+    try:
+        from cobs import cobs as cobs_mod
+    except ImportError:
+        print("ERROR: python cobs package not installed. Run: pip install cobs")
+        return
+
+    sizes = [0, 1, 10, 64, 254, 255, 256, 1024, 4096]
+    payloads = [
+        ("zeros", payload_zeros),
+        ("nonzero", payload_nonzero),
+        ("mixed", payload_mixed),
+    ]
+    iterations = 10_000
+
+    print(f"{'operation':<30} {'size':>6} {'iters':>7} {'total_ms':>10} {'per_op_us':>10} {'MB/s':>10}")
+    print("-" * 80)
+
+    for name, make in payloads:
+        for size in sizes:
+            data = make(size)
+
+            # Encode
+            elapsed = bench(cobs_mod.encode, data, iterations)
+            per_op_us = (elapsed / iterations) * 1_000_000
+            mbps = (size * iterations / elapsed / 1_000_000) if elapsed > 0 and size > 0 else 0
+            print(f"{'python-cobs encode/' + name:<30} {size:>6} {iterations:>7} {elapsed*1000:>10.2f} {per_op_us:>10.3f} {mbps:>10.1f}")
+
+            # Decode
+            encoded = cobs_mod.encode(data)
+            elapsed = bench(cobs_mod.decode, encoded, iterations)
+            per_op_us = (elapsed / iterations) * 1_000_000
+            mbps = (size * iterations / elapsed / 1_000_000) if elapsed > 0 and size > 0 else 0
+            print(f"{'python-cobs decode/' + name:<30} {size:>6} {iterations:>7} {elapsed*1000:>10.2f} {per_op_us:>10.3f} {mbps:>10.1f}")
+
+
+if __name__ == "__main__":
+    main()