Deep-copy mutable selector, schema, stylesheet, cascade, and computed-style values at public ownership boundaries while preserving function identity.\n\nVerified with make check: 57/57 ERT, WERROR byte compilation, checkdoc, and diff checks.
Create the lowercase ecss repository as a pure selector and CSS cascade package with no TP, Ebox, or historical ECSS dependency.\n\nVerified with 47 ERT tests, warning-free byte compilation, checkdoc, and git diff checks.
