Function joinPath

Append a structural suffix to a parent path. Mirrors the canonical joinPath used by the React renderers: when the suffix is omitted (e.g. transparent wrappers like union options) the parent path is returned unchanged so the child inherits the parent's id.

Suffixes are dot-joined except for bracketed array indices like [0] which append directly so tags + [0] becomes tags[0] rather than tags.[0].