docs(drift): document cnn drift evaluation outcome

Time-boxed review (30 min): upstream preserves only the outer
smartassetcontainer attribution paragraph while stripping the nested
iframe/script payload. Matching that behavior would need a site-specific
'attribution-bearing embed wrapper' heuristic, which would risk regressing
other widget/embed fixtures (calculators, social embeds, chart containers).
Decision: leave the drift in place; expand the rationale in
known-upstream-drift.json and README so the next reviewer does not
re-investigate from scratch.

Author: miclle

README.md

@@ -55,8 +55,13 @@ correct for this port or would break pinned fixtures.
   parser-inserted `<tbody>`. Many pinned fixtures contain explicit `<tbody>`
   markup, so this needs an implicit-vs-explicit table-section strategy before
   changing serialization.
-- `cnn`: current upstream keeps a small SmartAsset attribution block that this
-  port currently removes during cleanup.
+- `cnn`: current upstream keeps the outer `smartassetcontainer` (with only the
+  "Powered by SmartAsset.com" attribution paragraph) while stripping the nested
+  iframe/script payload. This port's embed cleanup removes the whole subtree.
+  Reviewed under a 30-minute time box: replicating upstream would require a
+  site-specific "attribution-bearing embed wrapper" heuristic that risks
+  regressing other widget/embed fixtures (calculators, social embeds, chart
+  containers), so the drift is intentionally left in place.
 
 ## Project Position
 

tools/known-upstream-drift.json

@@ -1,7 +1,7 @@
 {
   "cnn": {
     "fields": ["content", "textContent"],
-    "reason": "current upstream keeps a small SmartAsset attribution block that this port removes with the embed"
+    "reason": "current upstream keeps the outer smartassetcontainer (only the 'Powered by SmartAsset.com' attribution paragraph) while stripping the nested iframe/script payload. This port's embed cleanup removes the whole subtree. Replicating upstream would require a site-specific 'attribution-bearing embed wrapper' heuristic that risks regressing other widget/embed fixtures (calculators, social embeds, chart containers); reviewed and left as drift."
   },
   "firefox-nightly-blog": {
     "fields": ["byline", "content", "textContent"],