Snapshot URL datasets locally on add; preview tabular data as a table

docs/architecture/06-type-inference.md

@@ -183,7 +183,10 @@ how the UI shows **"N/A"** — see §3.2.
 
 ### 3.1 What gets profiled
 
-Profiling applies only to **tabular inline data**:
+Profiling applies to any **tabular payload**, whether pasted inline or fetched
+from a URL (the snapshot model stores a URL dataset's data locally, so it profiles
+through the same path as inline data — `snapshotFromText` shapes the fetched body,
+then `computeDatasetProfile` runs):
 
 - **JSON** that is an array of objects.
 - **CSV** (comma-separated, header row).
@@ -191,8 +194,9 @@ Profiling applies only to **tabular inline data**:
 
 Everything else is **not profiled**:
 
-- **URL datasets** — the library holds only the link, not the data, so there is
-  nothing to scan. Counts are `null` / N/A.
+- **Unfetched URL datasets** — a URL reference with no snapshot yet (e.g. one
+  migrated from an older record), so there is nothing to scan. Counts are `null` /
+  N/A until it is refreshed.
 - **Non-tabular data** — a single JSON object, TopoJSON, or anything we can't
   read as rows-of-columns. Counts are `null` / N/A.