Flink 1.18: connector and format parity matrix

Current behavior

Every connector and format StreamFusion ships builds and tests against Flink 1.18.1. What remains
is narrow: the Kafka connector's released 1.18 build targets Flink 1.18.0 rather than 1.18.1, and
that mismatch has not been stated as an admitted support boundary.

Shipped

• Per-line connector and format releases are selected by the flink-1.18 profile:
  flink-connector-kafka:3.2.0-1.18 with kafka-clients 3.4.0, paimon-flink-1.18:2.0.0, the
  1.18.1 formats, Calcite 1.32.0 and the protobuf 3.21.7 runtime that flink-protobuf:1.18.1
  generates against.
• All nine tagged module suites run on both lines in CI: Kafka, JSON, CSV, Raw, Avro,
  Avro-Confluent, Protobuf, Parquet and ORC.
• Paimon runs on both lines in CI for Parquet and ORC, and the upstream Paimon suite runs on both
  lines against its released 1.18 test contract.
• The upstream formats, parquet, orc and kafka suites run on both lines.
• Delta is decided: #197 audited the released legacy connector against Flink 1.18 and the
  Kernel-based implementation now belongs only to the 2.2 source root. There is no admitted
  native Delta connector on 1.18, and no 1.18 Delta payload is built or published. This is
  unavailable functionality, not a verified host fallback.
• Per-line format divergences are enumerated in docs/flink-compatibility.md: Avro legacy
  timestamp mapping only, Parquet map-key nullability in the footer, the 1.18 JSON decoder
  rejecting top-level arrays, the absent parser-selection option, and the older Jackson node
  factory's decimal spelling carried into JSON, CDC JSON and CSV encoders.

Remaining

• State the Kafka support boundary explicitly. flink-connector-kafka:3.2.0-1.18 is built
  against Flink 1.18.0 while StreamFusion targets 1.18.1, and it is the last Kafka connector
  published for that line. Record whether that combination is admitted, what was verified, and
  that no newer 1.18 connector will arrive. docs/connectors/kafka.md is the owning page.

Done when

The Kafka connector's 1.18 version boundary is documented as an admitted, verified combination
rather than an implicit one.

Split from #182.

Start with docs/connectors/kafka.md and compare its Flink 1.18 support statement with the boundary described in docs/flink-compatibility.md. Document the admitted and verified combination of flink-connector-kafka:3.2.0-1.18 with Flink 1.18.1, and state that no newer 1.18 Kafka connector will be published.