v0.3.0 — Multi-partition topics + resumable consumer groups

Multi-partition topics with key-based routing. A topic can now own N partitions,
each with its own writer task and log, so producers write in parallel. Records
route by key (hash(key) % partitions) so same-key records keep their order;
keyless records spread round-robin. A single consumer reads all partitions merged
into one stream. Ordering is guaranteed within a partition; cross-partition
order is intentionally undefined — the standard Kafka contract. Default partition
count is 1, so single-partition topics behave as before.

Added

• Resumable consumers via committed offsets (consumer groups, part 1).
  Kafko::consumer_for_group(topic, group) returns a consumer that resumes from
  the group's durably committed position instead of offset 0 — the
  "continue where it stopped after a restart" behaviour. Consumer::commit()
  persists the current per-partition read position (atomic temp + fsync + rename
  to <topic>/offsets/<group>, CRC-framed); Consumer::committed(partition) and
  Consumer::group() introspect. Distinct groups on a topic keep independent
  positions (durable pub/sub fan-out). A torn/corrupt offset file degrades to
  "start from 0", never an error. consumer_for stays anonymous (reads from 0;
  commit() is a no-op). New error KafkoError::InvalidGroupName.
  Single active consumer per group for now; multi-member partition assignment +
  rebalancing is a later slice.
• Kafko::create_topic_with_partitions(name, count) and
  create_topic_with_config_and_partitions(name, cfg, count) — create a topic
  with count partitions. count == 0 returns KafkoError::InvalidPartitionCount.
• Kafko::partition_count(name) -> Option<u32>.
• Topic — public type owning a topic's partitions and its routing (FNV-1a key
  hash + round-robin). Obtained via Kafko::topic.
• RecordPosition { partition, offset } — returned by all Producer send
  methods.
• Producer::send_to(partition, key, value) for explicit partition targeting,
  and Producer::partition_count().
• Consumer::next_with_position() (record + its RecordPosition),
  from_topic/from_topic_at, seek_all(offset), seek(partition, offset),
  position(partition), partition_count().

Changed (breaking)

• Producer::send / send_record now return RecordPosition instead of a
  bare u64 offset (an offset is only meaningful within a partition). send_batch
  / send_batch_records return Vec<RecordPosition>. Read .offset() /
  .partition() on the result.
• send_batch atomicity is now per-partition. A batch whose records route to
  different partitions is atomic within each partition, not across them. For a
  single-partition topic this is unchanged (one fully-atomic append).
• On-disk layout changed to <dir>/<topic>/<partition>/<segments>. Data
  directories written by kafko <= 0.2 (segments directly under the topic dir) will
  not open — Kafko::open returns KafkoError::InvalidTopicLayout. There is no
  automatic migration.
• Kafko::topic returns Arc<Topic> (was Arc<Partition>).
• Producer::new takes Arc<Topic>; Consumer is built from a Topic
  (from_topic / from_topic_at) and Consumer::seek now takes a partition
  index (use seek_all for the old whole-stream behaviour).