v0.1.1 — send_batch + LZ4 OOM fix

Security

• Fixed: LZ4 decompress OOM (DoS). Records with Compression::Lz4 whose
  payload had an adversarial 4-byte LE size prefix could coerce
  lz4_flex::decompress_size_prepended into a ~4 GiB Vec::with_capacity
  call before the compressed bytes were validated, OOM-crashing the host
  process. Triggerable by any read of a corrupt or adversarial segment file.
  Fixed by bounding the claimed decompressed size at 16 MiB before delegating
  to lz4_flex (matching the existing Zstd cap). Discovered via the new
  cargo-fuzz decode_record_structured target. v0.1.0 is yanked.

Added

• Producer::send_batch(Vec<(Option<Bytes>, Bytes)>) and
  Producer::send_batch_records(Vec<Record>) — atomic, single-mpsc-round-trip
  batched appends. ~10× throughput vs a loop of send() at N = 1024
  (2.53 M rec/s vs 252 K rec/s, criterion-measured, 256 B records, in-process).
• Partition::append_batch(Vec<Record>) -> Result<Vec<u64>> — the actor
  primitive underlying Producer::send_batch.
• Two new criterion benches: send_batch (batch-size × codec sweep),
  config_sweep (segment_size_threshold / index_interval / preset configs).
• Cargo-fuzz scaffold under fuzz/ with five targets: decode_record,
  decode_record_structured, recovery_torn_tail, sparse_index_parse,
  log_op_sequence. WSL2 setup documented in the README.
• Proptest coverage expanded to all three compression codecs, the
  SparseIndex::lookup invariant, Log append-then-read invariant, and
  Log rotation under random LogConfig.
• "Performance recipes — pick once, ship it" section in both READMEs:
  decision table + four code recipes + "what NOT to tune" guidance.

Fixed

• Segment::would_overflow no longer reports overflow on an empty segment.
  Previously the first append (or batch) whose wire size exceeded
  segment_size_threshold against a fresh empty log triggered rotation,
  which then tried to create a new segment at next_offset = 0 and failed
  with ErrorKind::AlreadyExists. The size cap is now correctly a soft
  target on segments with content; empty segments always accept the first
  write. Reachable under default config only at > 1 GiB single records;
  reachable under any custom small-threshold config.

Changed

• Internal constant ZSTD_DECOMPRESS_MAX_SIZE renamed to DECOMPRESS_MAX_SIZE
  and now applies uniformly to LZ4 and Zstd.
• README: Producer::send_batch moved from v0.2 roadmap to v0.1 list. The
  kafko-http /produce_batch bullet was dropped from the roadmap — it's
  an improvement for the measurement harness, not the library.