feat: add live surviving rows preview in match group editor

Shows a read-only preview of the output rows below the editor,
updating as checkboxes and column dropdowns are changed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/gui/components.py

@@ -422,10 +422,11 @@ def match_group_card(
                     f"{', '.join(differing_cols)}"
                 )
 
-            # Status
+            # Status + surviving rows preview
             if len(checked) == 0:
                 st.warning("Select at least one row to keep.")
-            elif len(checked) == n_rows:
+            else:
+                if len(checked) == n_rows:
                     st.caption("Keeping all rows (no duplicates removed)")
                 elif len(checked) == 1:
                     st.caption(
@@ -438,6 +439,17 @@ def match_group_card(
                         f"removing {n_rows - len(checked)}"
                     )
 
+                # Build preview of surviving rows with edits applied
+                checked_positions = [
+                    i for i, idx in enumerate(group.row_indices)
+                    if idx in checked
+                ]
+                preview = edited.iloc[checked_positions].drop(
+                    columns=["Keep"],
+                ).reset_index(drop=True)
+                st.markdown("**Surviving rows preview:**")
+                st.dataframe(preview, use_container_width=True, hide_index=True)
+
             # Confirm
             def _on_confirm(
                 g=gid, indices=list(group.row_indices),