[Architecture Review][P0] Partition eventmesh-common into internal packages

[qqeasonchen]

Execution meta — see plan: #5296

• Phase: 4 — boundary decoupling
• Severity: P0
• Depends on: [Architecture Review][P2] Gradle dependency guardrails (architectureCheck) #5305
• Blocks: [Architecture Review][P0] Single protocol path (HTTP + CloudEvents) and ingress/egress adapters #5299

Problem

eventmesh-common is an oversized "kitchen-sink" module. It exposes CloudEvents, Netty, gRPC, HTTP client, Jackson, Fastjson, Disruptor, and more. SPI, Storage API, and Protocol API all depend on it.

Risks

• Plugin developers are forced to pull in many unrelated dependencies.
• SDK and Runtime are prone to dependency conflicts.
• Kernel, protocol, network, serialization, and utilities cannot evolve independently.

Proposed direction

Keep eventmesh-common as a single module for now, but partition it into clear internal packages — no new modules, since the module count is already too high:

org.apache.eventmesh.common.model    EventMeshFrame, topic, offset, error model, base value objects
org.apache.eventmesh.common.wire     Frame codec / WireCodec
org.apache.eventmesh.common.config   configuration model + validation
org.apache.eventmesh.common.util     utility classes with no infrastructure dependency

• Make eventmesh-spi, the Storage API, and the Protocol API depend only on the model / wire packages, not on the whole eventmesh-common (keep the API surface small even if physically one module).
• Enforce package boundaries with an ArchUnit rule (see [Architecture Review][P2] Gradle dependency guardrails (architectureCheck) #5305).

A physical module split is a follow-up only — do it only if a hard dependency cut is needed so plugin authors stop pulling Netty / gRPC transitively.

Acceptance criteria

• model and wire packages are free of Netty / gRPC / HTTP / Jackson / Fastjson / Disruptor imports.
• SPI, Storage API, and Protocol API reference only model / wire packages (enforced by ArchUnit).
• Migration is staged (no single giant-bang PR); deprecation notes added to eventmesh-common.

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Part of the Architecture Review.