Subscriptions silently drop when agent branch doesn't match client distro — undocumented #270

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 <firmware_pub_topic> — works, data is visible.
4. From host, ros2 topic pub <firmware_sub_topic> .... 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.