Subscriptions silently drop when agent branch doesn't match client distro — undocumented

還沒有人認領這個 Issue。

Hi,
Spent a while chasing a bug that turned out to be agent ↔ client version mismatch.

- Hardware description: STM32G491 (Cortex-M4), custom board, serial UART transport
- RTOS: Zephyr 4.1.x
- Installation type: modules (`micro_ros_zephyr_module`, jazzy branch)
- Version or commit hash: client on `jazzy` branch; agent `6.0.0`

#### Steps to reproduce the issue

1. Build a Zephyr firmware client from the `jazzy` branch of `micro_ros_zephyr_module`. Add at least one publisher and one subscription on the same node.
2. Run micro_ros_agent 6.0.0 (built from source) with serial transport, pointed at the firmware.
3. From host (ROS 2 jazzy), `ros2 topic echo ` — works, data is visible.
4. From host, `ros2 topic pub ...`. Subscription callback never fires on the firmware. Agent log at `-v6` shows DATA submessages being forwarded toward the client.
5. Stop everything. `git clone` `micro-ROS-Agent`, `git checkout jazzy`, build from source, restart with the same args.
6. Subscription callback fires correctly on the first try.

#### Expected behavior

Either:
- Tagged agent releases interoperate with any matching client, **or**
- A loud error / log line at session-establish when agent and client XRCE protocol versions don't line up.

#### Actual behavior

Subscription messages are silently dropped on the way to the client. No error, no warning, no log line at any verbosity. Publishers in the same direction work fine, so it looks superficially like a working session.

#### Additional information

The README doesn't mention that the agent branch has to match the client's ROS distro at runtime. Two asks:

1. **Docs:** add a compatibility note to the README — the agent must be built from the branch matching the client's distro (`humble`, `jazzy`, `rolling`). Happy to PR this.
2. **Optional:** at session-establish, log both the agent's and the client's claimed XRCE protocol version so a mismatch is loud rather than silent.

從儲存庫的 README 開始，檢視其中關於 agent/client 版本的指引，並將回報的 humble、jazzy 和 rolling 分支作為相容性案例。新增一則明確說明：agent 分支必須與 client 的 ROS 發行版相符；當這項要求和靜默失敗的風險都已清楚且沒有歧義時，文件工作即告完成。issue 中未指定任何測試。

由索引模型根據 Issue 內容生成。