Restructure plots project to consolidate shared functionality #470

The legacy and vizapp subdirectories contain significant duplicated code: compute.py, tools.py/utils.py, gwtc3.py, normalize_path(), and VETO_DEFINER_FILE/GATE_PATHS are all fully or nearly identical across both. As new plot types are added, they risk being duplicated again under the current structure. Establish a shared module (e.g., plots/shared/ or plots/core/) for plot computation and utilities that both the CLI and vizapp import from. Rename legacy/ to reflect its role as the non-interactive CLI interface.