Align internal documentation and maintained examples

docs/internal/stage-prepare.md

@@ -1,17 +1,18 @@
 # Stage: prepare
 
 ## Purpose
+
 Materialize canonical current-session inputs before processing stages.
 
 ## Inputs
-- resolved `campaign.yml`, `session.yml`, and pipeline config
+
+- resolved campaign, session, and pipeline configuration
 - stable input files (`speakers`, `autocorrect`, `glossary`, `players`, `party`)
-- audio source:
-  - local `audio_dir`/`audio_files`, or
-  - S3 `audio_s3.prefix`
+- one resolved local or S3 audio source
 - enabled configured artifact input requirements for previous-session sources
 
 ## Outputs
+
 - `inputs/campaign.yml`
 - `inputs/session.yml`
 - `inputs/pipeline.resolved.yml`
@@ -26,6 +27,7 @@ Materialize canonical current-session inputs before processing stages.
 - deterministic `manifest.inputs` entries (checksums + provenance)
 
 ## Key Behavior
+
 - validates required config/store state.
 - enforces local audio vs S3 audio mutual exclusivity.
 - materializes S3 audio through spool/cache-aware logic.
@@ -39,6 +41,20 @@ Materialize canonical current-session inputs before processing stages.
 Required previous-session inputs fail when unavailable; optional missing inputs are skipped.
 
 ## Invariants
+
 - only `prepare` hydrates canonical `previous/` cache state.
-- managed previous artifacts are stored under `previous/artifacts/**` without duplicate `artifacts/artifacts/` nesting.
+- managed previous artifacts are stored under `previous/artifacts/**` without
+  duplicate `artifacts/artifacts/` nesting.
 - `manifest.inputs` ordering is deterministic (`kind`, `path`).
+
+## Related Contracts And Tests
+
+- [Configuration](../config.md) owns audio selection, stable input fields, and
+  previous-session settings.
+- [Operations](../operations.md) owns physical input, audio, spool, cache, and
+  previous-state layout.
+- [Storage Internals](storage.md) and [Artifact Internals](artifacts.md) explain
+  the internal collaborators.
+- Implementation and tests: `internal/stage/prepare.go`,
+  `internal/stage/prepare_test.go`, `internal/audio/s3_audio_test.go`,
+  `internal/previouscache/*_test.go`