Add a nil-safe generic gg.Clone[T](*T) *T to utils/gg alongside the existing
V and If helpers, and route the lyrics package's bespoke pointer-copy helpers
through it: ptrOrNil (model/lyrics.go) and cloneTTMLCue (model/lyrics_ttml.go)
are removed in favor of gg.Clone. Also replaces the local positiveOrDefault
helper with gg.If at its three TTML timing-param call sites.
This removes duplicated single-purpose plumbing in favor of shared, generic
utilities. Behavior is unchanged. Adds tests for gg.Clone covering copy,
non-aliasing, and nil input.
c66d92c724