Download S3 audio during prepare

README.md

@@ -49,7 +49,7 @@ YAML decoding is strict (`KnownFields(true)`), so unknown fields fail fast.
 
 ## Storage And Archive Foundations
 
-Narratio now includes configuration and path-model foundations for future S3 audio input and archive support.
+Narratio now includes configuration and path-model foundations for archive support, plus implemented prepare-stage S3 audio input.
 
 Implemented foundations:
 
@@ -58,7 +58,7 @@ Implemented foundations:
 - `pipeline.archive` config shape (`enabled`, `upload_run`, `promote_artifacts`)
 - promotion-rule validation (`from`/`to` required, relative-only paths, traversal rejected)
 - `session.campaign` requirement for campaign-aware path construction
-- optional `session.inputs.audio_s3.prefix` modeling (not executed yet)
+- optional `session.inputs.audio_s3.prefix` modeling and prepare-stage S3 audio download
 - run ID generation and S3/local path helper foundations
 - manifest run/path identity fields
 
@@ -79,10 +79,11 @@ Current boundaries:
 - `audio_dir`/`audio_files` and `audio_s3` are mutually exclusive
 - real S3-compatible backend now exists in the storage adapter package
 - storage backend tests use fake storage and do not require live S3
-- no prepare-stage S3 list/download behavior yet
 - no archive-stage S3 upload/promotion behavior yet
 - no `current/manifest.json` or `current/run_id.txt` uploads yet
 
+S3 input details and current boundaries are documented in [docs/s3-audio-input.md](docs/s3-audio-input.md).
+
 ## Remote Storage Backend
 
 Narratio includes an object-store backend layer for future prepare/archive work:
@@ -104,7 +105,8 @@ Key invariant:
 
 Current boundary:
 
-- this backend layer is implemented but not yet wired into prepare-stage audio retrieval or archive-stage uploads
+- `prepare` uses `List` + `Download` through the backend when `session.inputs.audio_s3` is configured
+- archive-stage uploads/promotion are still not implemented
 
 ## Canonical Stage Order