Implemented shared S3 audio caching for prepare and restore --include-audio

docs/internal/stage-prepare.md

@@ -55,6 +55,8 @@ Does not own:
 - `session.inputs.audio_s3.prefix`
 - `pipeline.workspace.root`
 - `pipeline.spool.root`
+- `pipeline.cache.root`
+- `pipeline.cache.s3_audio`
 - `pipeline.storage.s3.bucket`
 - `pipeline.storage.s3.root_prefix`
 - `pipeline.scriptorium.artifacts.<name>.enabled`
@@ -73,6 +75,7 @@ Does not own:
 ## State and manifest behavior
 - Ensures workspace layout exists.
 - Materializes canonical input files and audio files.
+- For S3 audio, uses run-scoped spool for active downloads and durable cache for reusable audio files; cache hits copy directly to work audio without downloading the object again.
 - Records `inputs/session.yml` provenance as local `session_config` or remote `session_config.s3`.
 - Resolves campaign-provided stable input paths relative to `campaign.yml`.
 - Resolves session-provided stable input overrides relative to `session.yml`.
@@ -86,6 +89,7 @@ Does not own:
   - records hydrated previous inputs in `manifest.Inputs` with source `previous_session_archive.current`.
 - If no canonical previous-session requirements exist, prepare does not manage `previous/`.
 - `manifest.Inputs` is sorted deterministically by `(kind, path)`.
+- S3 audio `manifest.Inputs` retain S3 provenance and include `cache_path`; `spool_path` is present only when the current prepare invocation downloaded the file.
 
 ## Required and optional previous-session behavior
 - `previous_session_id` unset: