Require manifest-backed local artifacts

docs/internal/artifacts.md

@@ -20,8 +20,8 @@ The internal registry recognizes these stable built-in source IDs:
 - `narratio.transcript.final_trimmed_markdown`
 - `narratio.bounds.session`
 
-Registry entries bind each ID to its producer, output kind, canonical fallback,
-and content validator. The focused stage documents own their input/output flow;
+Registry entries bind each ID to its producer, output kind, and content
+validator. The focused stage documents own their input/output flow;
 [Configuration](../config.md) owns where operators may select these IDs.
 
 ## Configured, Extraction, And Previous-Session Sources
@@ -91,8 +91,13 @@ Current provenance values:
 
 Built-ins:
 
-1. manifest producer outputs (when present)
-2. canonical session-path fallback
+- resolve only from the current manifest's succeeded registered producer;
+- require exactly one output with the registered source ID and output kind, a
+  safe session-rooted path, producer run ID, canonical checksum, and valid
+  content; and
+- verify the registered checksum against the durable bytes. Incidental files,
+  canonical filenames, duplicate outputs, failed/stale producer records, and
+  malformed evidence are unavailable.
 
 Configured sources (`narratio.artifact.*`):