The content-addressed longform_cache/ accumulated uncompressed chapter WAVs
across every render with no bound (a review finding). Add prune_cache_dir() —
LRU-by-mtime eviction down to a 2 GB ceiling (OMNIVOICE_LONGFORM_CACHE_MAX_GB);
best-effort, never raises. Called at the start of each render job, before its
chapters are written, so the fresh ones are never the eviction target.
Tests: under-cap no-op, evicts-oldest-keeps-newest, missing-dir safe. 38 green.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>