individual run evidence, retained summaries, and trend comparison

docs/ARCHITECTURE-BLUEPRINT.md

@@ -398,7 +398,8 @@ Builds human and machine-readable outputs:
 ### Retention Index
 
 Keeps compact summaries over time while allowing raw artifact retention to be
-bounded by policy.
+bounded by policy. The first implementation writes `retention-summary.json` for
+each run and can build a trend summary grouped by target and assessment profile.
 
 ## Extension Archetypes
 

docs/schemas/trend-summary.schema.json

@@ -0,0 +1,20 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "Guide Board Trend Summary",
+  "type": "object",
+  "additionalProperties": false,
+  "required": [
+    "id",
+    "created_at",
+    "runs_dir",
+    "run_count",
+    "groups"
+  ],
+  "properties": {
+    "id": { "type": "string" },
+    "created_at": { "type": "string" },
+    "runs_dir": { "type": "string" },
+    "run_count": { "type": "integer" },
+    "groups": { "type": "array", "items": { "type": "object" } }
+  }
+}