examples/microros_sub: Add minimal Int32 subscriber example.

[arjav1528]

Summary

Phase 7 of the micro-ROS on NuttX integration. Adds apps/examples/microros_sub,
a minimal Int32 subscriber that mirrors the microros_pub example merged in
#3512 for the receive direction.

The example creates a nuttx_sub_node, subscribes to /nuttx_sub with
std_msgs/msg/Int32, registers a callback via rclc_executor_add_subscription,
and enters rclc_executor_spin. Each received value is printed via NSH. It
exercises the rclc_executor + subscription callback path on top of the
NuttX-native microros_transport library that was wired up in Phase 6.

Depends on:

• system/microros: Add UDP and serial custom transports with publisher example #3512 (transport + pub example) — merged
• tools/nxstyle: Whitelist ROS 2 message type identifiers. nuttx#19001 (CMake plumbing) — merged

Impact

• User: new opt-in EXAMPLES_MICROROS_SUB Kconfig entry under
  Application Configuration > Examples, gated on SYSTEM_MICROROS.
  Default n; existing configs are unaffected.
• Build: no changes to the libmicroros build pipeline; reuses the existing
  microros_transport library and SYSTEM_MICROROS include layout.
• Hardware: none. Sim and any board with the existing micro-ROS transport
  enabled (UDP via NuttX sockets or serial via TERMIOS) can run the example.
• Docs / security / compatibility: none.

Testing

Host: Ubuntu 22.04.5 LTS (jammy), x86_64, Linux 6.8.0-1055-aws,
gcc 11.4.0, AWS EC2 m7i-flex.large (2 vCPU, 7.6 GB RAM).

Target: NuttX sim:nsh on Linux, UDP transport over TAP.

1. checkpatch

$ ./tools/checkpatch.sh -f ../apps/examples/microros_sub/microros_sub_main.c
(clean)

2. Build with pub + sub both enabled

$ tools/configure.sh sim:nsh
$ cat ~/microros_extra_config >> .config
$ make olddefconfig
$ make -j2
...
LD: nuttx
SIM elf with dynamic libs archive in nuttx.tgz

Result: nuttx ELF produced, both microros_pub_main and microros_sub_main
linked.

3. Sub-only build (pub disabled)

Verifies the new example does not regress when the existing publisher example
is turned off and that microros_transport still links cleanly with just the
subscriber as a consumer.

$ kconfig-tweak --disable EXAMPLES_MICROROS_PUB
$ kconfig-tweak --enable  EXAMPLES_MICROROS_SUB
$ make olddefconfig && make -j2
...
LD: nuttx
SIM elf with dynamic libs archive in nuttx.tgz
$ nm nuttx | grep -E 'microros_sub_main|sub_callback'
00000000400a9d89 T microros_sub_main
00000000400a9d30 t sub_callback

4. End-to-end runtime against a micro-ROS agent

Agent (host, separate terminal):

$ docker run -it --rm --network host microros/micro-ros-agent:jazzy \
      udp4 --port 8888
[INFO] [Root.Agent]: Running.

NuttX sim:

nsh> ifconfig
eth0    Link encap:Ethernet HWaddr 42:0a:bc:cd:ef:02 at UP
        inet addr:192.168.10.2 DRaddr:192.168.10.1 Mask:255.255.255.0
nsh> microros_sub &
microros_sub: starting
microros_sub: spinning on /nuttx_sub

Host publishes (in another terminal):

$ ros2 topic pub --once /nuttx_sub std_msgs/msg/Int32 "{data: 42}"
$ for v in 100 200 300 400 500; do
    ros2 topic pub --once /nuttx_sub std_msgs/msg/Int32 "{data: $v}"; done

NuttX sim output:

microros_sub: got 42
microros_sub: got 42
microros_sub: got 42
microros_sub: got 42
microros_sub: got 42
microros_sub: got 42
microros_sub: got 42
microros_sub: got 100
microros_sub: got 200
microros_sub: got 300
microros_sub: got 400
microros_sub: got 500

All values delivered in order to the registered callback via
rclc_executor_spin, confirming the agent → client direction of the
NuttX-native transport plus the rclc_executor subscription path works
end-to-end.

[lupyuen]

FYI: I restarted the CI Build to fix the Docker Image

• ci/docker: bump risc-v toolchain gcc14 nuttx#19035