Download S3 audio during prepare

architecture.md

@@ -41,7 +41,6 @@ Implemented:
 Still placeholder/future:
 
 - `archive` stage behavior
-- prepare-stage S3 audio download behavior
 - archive-stage S3 upload/promotion behavior
 - `notify` stage behavior
 - additional Scriptorium artifact types beyond `session_recap`
@@ -152,9 +151,9 @@ Session input foundations:
 
 - `session.campaign` is required
 - local audio remains supported through `session.inputs.audio_dir` or `session.inputs.audio_files`
-- optional S3 audio input shape is modeled as `session.inputs.audio_s3.prefix`
+- optional S3 audio input shape is `session.inputs.audio_s3.prefix`
 - `audio_dir`/`audio_files` and `audio_s3` are mutually exclusive
-- S3 input execution (object listing/downloading) is not implemented yet
+- when `audio_s3` is configured, `prepare` lists and downloads `.flac` objects through the object-store backend
 
 Cross-config validation scope:
 
@@ -168,7 +167,21 @@ Remote object-store backend scope:
 - S3 key/session path semantics remain outside the backend, with this invariant:
   - callers pass full bucket-relative object keys
   - backend methods do not prepend `root_prefix` or infer campaign/session/run paths
-- the backend layer is available for future prepare/archive usage, but no stage currently invokes `List`/`Download`/`Upload`/`Exists`
+- `prepare` now uses object-store `List` and `Download` for S3 audio input
+- archive upload/promotion behavior using `Upload`/`Exists` remains future work
+
+Prepare S3 audio behavior (implemented):
+
+- compute session prefix as `{root_prefix}/campaigns/{campaign}/sessions/{session_id}/`
+- resolve `session.inputs.audio_s3.prefix` under that session prefix
+- list objects under the computed audio prefix and filter `.flac` keys
+- fail clearly when no `.flac` objects are found
+- download selected objects to spool audio path:
+  - `{spool.root}/{campaign}/{session_id}/{run_id}/audio/`
+- materialize audio files into workdir audio path:
+  - `{workspace.root}/work/{campaign}/{session_id}/{run_id}/audio/`
+- record S3 provenance in manifest input records (bucket/key/metadata/local paths/checksum)
+- no AWS SDK types are used in stage code; storage implementation details stay in storage adapter packages
 
 `pipeline.scriptorium` is optional. Existing pipelines without Scriptorium continue to work.