[kafka] Establish compatibility framework

[gyang94]

Purpose

Linked issue: #4185

This is PR 1 of 4 for the Basic Produce compatibility milestone. It establishes the protocol and module foundations required by the later Produce PRs without introducing the Produce implementation itself.

Brief change log

• Add the Kafka protocol plugin and request/response dispatch framework.
• Add API version negotiation and typed API registration.
• Add topic-to-table mapping plus metadata and topic administration backends.
• Add the Kafka compatibility module to the build and document the proposed architecture.

Tests

• Added unit coverage for API registration and dispatch.
• Added unit coverage for Kafka topic mapping.
• Reactor build, Checkstyle, Spotless, and RAT validation passed while rebuilding the stack.

API and Format

This adds internal Kafka compatibility interfaces and wiring. It does not change the existing Fluss public API or storage format.

Documentation

Adds the Kafka protocol compatibility framework proposal.

Stack

• PR 1/4 — this PR
• PR 2 — basic Produce compatibility
• PR 3 — SASL/PLAIN authentication
• PR 4 — schema-aware Produce transcoding

Generative AI disclosure: Codex (GPT-5.6) assisted with implementation and commit organization. Human review is required before merge.