Adds a content-addressed segment cache (segment_cache_key + SegmentCache,
cache_dir/segments/) under the existing chapter cache: a changed chapter now
reuses every untouched span's WAV and synthesizes only the edited/missing
ones, and an interrupted chapter render resumes from the segments that already
finished (each persists the moment it renders). The chapter key derivation is
unchanged so on-disk caches from released versions keep hitting, a fully-
unchanged chapter never touches segment files, and prune_cache_dir now walks
both layers so one byte cap bounds the whole cache. Chapter SSE events gain
additive segments/cached_segments counts.
Co-authored-by: mergetest <test@local>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
