Track hidden rcs_zed dependency on rcs_realsense calibration

[juelg]

Summary

rcs_zed currently imports FR3BaseArucoCalibration from rcs_realsense.calibration in extensions/rcs_zed/src/rcs_zed/utils.py, but extensions/rcs_zed/pyproject.toml does not declare rcs_realsense as a dependency.

That means rcs_zed is not actually installable as an independent extension for users who only install the ZED module.

Current state

• default_zed(...) uses FR3BaseArucoCalibration
• default_zed_dummy_calibration(...) exists, but does not remove the hidden dependency from the main helper
• the current Docker/dev setup masks the issue because multiple extensions are installed together

Why this matters

Users installing rcs_zed on its own can hit an import-time failure due to the undeclared rcs_realsense dependency.

Possible fixes

1. Move the shared calibration strategy into core rcs and import it from there in both extensions
2. Or, if that refactor should happen later, make default_zed(...) stop depending on rcs_realsense for now
3. As a temporary packaging fix, explicitly declare rcs_realsense as a dependency of rcs_zed

Context

Observed while reviewing PR #277.