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>

justfile

@@ -0,0 +1,64 @@
+# μCOBS development tasks
+
+# default: list all recipes
+default:
+    @just --list
+
+# run ALL quality gates (tests, clippy, fmt check, doc, fuzz, miri)
+test: test-unit test-clippy test-fmt test-doc test-fuzz test-miri
+
+# quick compile check (no tests)
+check:
+    cargo check
+
+# unit + integration + proptest tests (via nextest for parallelism)
+test-unit:
+    cargo nextest run
+    cargo test --doc
+
+# clippy with all warnings as errors
+test-clippy:
+    cargo clippy -- -D warnings
+
+# check formatting
+test-fmt:
+    cargo fmt -- --check
+
+# doc build (catches broken doc links)
+test-doc:
+    cargo doc --no-deps
+
+# fuzz all targets (60s each, requires nightly + cargo-fuzz)
+test-fuzz:
+    cd fuzz && cargo +nightly fuzz run fuzz_decode -- -max_total_time=60
+    cd fuzz && cargo +nightly fuzz run fuzz_roundtrip -- -max_total_time=60
+    cd fuzz && cargo +nightly fuzz run fuzz_small_dest -- -max_total_time=60
+
+# run tests under miri (catches UB, requires nightly + miri component)
+# proptest is excluded — its file-persistence layer calls getcwd which miri does not support
+test-miri:
+    cargo +nightly miri test -- --skip proptests
+
+# run criterion benchmarks (ucobs vs cobs vs corncobs)
+bench:
+    cargo bench
+
+# run python cobs benchmarks (requires: pip install cobs)
+bench-python:
+    python3 bench_python.py
+
+# run all benchmarks (rust + python)
+bench-all: bench bench-python
+
+# format code
+fmt:
+    cargo fmt
+
+# build in release mode
+build:
+    cargo build --release
+
+# clean all build artifacts
+clean:
+    cargo clean
+    rm -rf fuzz/target fuzz/corpus fuzz/artifacts