[Architecture Review][P0] Decouple eventmesh-runtime responsibilities via 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][P1] A2A must not form a parallel Runtime #5302

Problem

eventmesh-runtime carries too many responsibilities and couples the core delivery path too tightly to infrastructure. A single module currently bundles:

• core delivery logic (UniRuntime, UniIngressService, ReliableDispatcher);
• HTTP / SSE / WebSocket servers;
• Nacos cluster metadata;
• RocksDB offset store;
• protocol plugin assembly;
• Connector scheduling;
• session / agent capabilities.

For example, EventMeshApplication directly creates NacosMetaStore, RocksDBOffsetStore, and the HTTP/WS servers, then assembles the Runtime. eventmesh-runtime/build.gradle also depends directly on Netty, RocksDB, Nacos, and concrete protocol implementations.

Risks

• The Runtime core cannot be embedded, unit-tested, or swapped easily.
• Any protocol or infrastructure change can ripple into the core delivery path.
• The module's claimed "minimal dependency surface" does not match its real dependencies.

Proposed direction

Keep eventmesh-runtime as a single module, but enforce an internal package partition — no new modules, since the module count is already too high:

org.apache.eventmesh.runtime.engine      core delivery: UniRuntime / UniIngressService
org.apache.eventmesh.runtime.bootstrap   configuration reading + assembly only
org.apache.eventmesh.runtime.http        HTTP / SSE / WS servers
org.apache.eventmesh.runtime.state       RocksDB-backed state (behind interfaces)
org.apache.eventmesh.runtime.meta        Nacos-backed meta (behind interfaces)

• UniRuntime / UniIngressService depend only on interfaces (OffsetStore, MetaStore, PushChannel, MeshStoragePlugin); the concrete NacosMetaStore / RocksDBOffsetStore / Netty implementations live in their own packages.
• EventMeshApplication only reads configuration and wires components; it must not host core business decisions.
• Enforce the package boundaries with an ArchUnit rule (see [Architecture Review][P2] Gradle dependency guardrails (architectureCheck) #5305) rather than Gradle module boundaries.

A physical module split is not required at this stage — it is a follow-up only if hard compile-time isolation is needed later.

Acceptance criteria

• UniRuntime / UniIngressService have no compile-time dependency on Netty, RocksDB, Nacos, or a concrete protocol plugin.
• The engine package can be unit-tested without starting an HTTP server or Meta store.
• bootstrap has no delivery logic.
• An ArchUnit package-dependency rule (from [Architecture Review][P2] Gradle dependency guardrails (architectureCheck) #5305) blocks engine → http / state / meta imports.

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Part of the Architecture Review.