Add headless world launch option

[yeseorizi]

Summary

Add a headless argument to the standalone dave_world.launch.py entry point.

Problem

DAVE's robot, sensor, and object launch files already accept headless, but the world-only launch exposed only world_name and verbose. A world therefore could not be started server-only through this entry point without bypassing the DAVE launch file.

Baseline --show-args output contained:

world_name
verbose

Change

• Declare headless with a default of false.
• Append Gazebo's -s server-only flag when headless:=true.
• Preserve the existing default launch behavior.

Example:

ros2 launch dave_demos dave_world.launch.py \
  world_name:=dave_ocean_waves headless:=true

Validation

The exact candidate package was installed in the ARM64 DAVE environment with ROS 2 Lyrical and Gazebo Jetty.

A minimal test world was launched through the modified entry point:

ros2 launch dave_demos dave_world.launch.py \
  world_name:=pr58_headless headless:=true verbose:=true

Observed process:

gz sim -r .../pr58_headless.world --verbose -s --force-version 10

Validation results:

• headless appears in --show-args: PASS
• /world/pr58_headless/control service became available: PASS
• Gazebo process received -s: PASS
• No Gazebo graphical client was started: PASS

Repository checks:

python3 -m py_compile examples/dave_demos/launch/dave_world.launch.py
# Passed

pre-commit run --all-files
# Passed

git diff --check
# Passed

Checklist

• Change is limited to the standalone world launch
• Existing default remains unchanged
• Exact candidate package installed
• Server-only runtime path verified
• Repository hooks passed