Fix --package runtime dependency resolution for user-prefix Chrono and ROS-provided urdfdom

[Copilot]

install(TARGETS ... RUNTIME_DEPENDENCIES) uses file(GET_RUNTIME_DEPENDENCIES), which ignores LD_LIBRARY_PATH by design and only searches the binary's INSTALL_RPATH ($ORIGIN/../lib, empty at install time) plus system paths. Packaging therefore fails to resolve libChrono_core.so (user prefix) and liburdfdom_model.so.4.0 (ROS, in lib/x86_64-linux-gnu/), even though the build and link succeed.

Changes (top-level CMakeLists.txt)

• Search the Chrono library directory explicitly — new SEASTACK_RUNTIME_DEP_DIRS cache variable for user-supplied paths, augmented with the directory derived from Chrono::Chrono_core's IMPORTED_LOCATION_RELEASE/IMPORTED_LOCATION, passed as DIRECTORIES to both the APPLE and non-APPLE install calls. Chrono is SEA-Stack's own dependency and stays in the bundle.
• Exclude ROS from the bundle — ^/opt/ros/ added to POST_EXCLUDE_REGEXES. urdfdom arrives transitively through Chrono::Chrono_parsers; ROS is an external runtime requirement, and vendoring it would make the archive distro-specific.
• _seastack_dep_dirs is unset alongside _seastack_dep_exclude.

install(TARGETS run_seastack
    RUNTIME_DEPENDENCIES
        DIRECTORIES ${_seastack_dep_dirs}
        POST_EXCLUDE_REGEXES ${_seastack_dep_exclude}
    CONFIGURATIONS Release
    RUNTIME DESTINATION bin COMPONENT runtime
    LIBRARY DESTINATION lib COMPONENT runtime)

With no Chrono target and an empty cache variable, DIRECTORIES expands to nothing, leaving the default system-Chrono path behaviour unchanged.

Note: a full --package run could not be exercised here (no Eigen3/Chrono in the environment); the install block was validated standalone, including the empty-DIRECTORIES case.

• Fixes --package fails to resolve runtime dependencies when Chrono is in a user prefix and urdfdom comes from ROS #19