[Architecture Review][P2] Gradle dependency guardrails (architectureCheck)

[qqeasonchen]

Execution meta — see plan: #5296

• Phase: 0 — guardrails & direction
• Severity: P2
• Depends on: (none)
• Blocks: [Architecture Review][P0] Decouple eventmesh-runtime responsibilities via internal packages #5297, [Architecture Review][P0] Partition eventmesh-common into internal packages #5298, [Architecture Review][P0] Single protocol path (HTTP + CloudEvents) and ingress/egress adapters #5299

Problem

Module dependencies keep expanding, and the Runtime's E2E tests already carry complex classpath isolation to support RocketMQ 4, RocketMQ 5, and Kafka.

Proposed direction

• Add an architectureCheck that blocks SPI/API from depending on the Runtime, broker clients, or Netty/gRPC implementations.
• Use a baseline to record existing coupling, but forbid new coupling.
• Move broker E2E tests into a dedicated integration-test module or profile so they do not pollute the core modules' test classpath.
• Eliminate the baseline gradually, not as a permanent exemption.

Acceptance criteria

• architectureCheck fails the build on new SPI/API → Runtime/broker/Netty/gRPC dependencies.
• A baseline file records existing coupling; the baseline only shrinks over time.
• Broker E2E tests are isolated in an integration-test module/profile.

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Part of the Architecture Review.