blog(derived-variables): the images for the computed-column styling

The four screenshots were taken on the mocked estate before the EDIT grid
marked derived columns apart from ordinary read-only ones.  Re-taken against a
real SAS backend, so they show what ships: the fx badge, the tinted header and
cells, and the edge that opens the computed block.

The read-only sentence now says what the marking means, which is the one claim
the old images made ambiguously.

content/blog/derived-variables/index.md

@@ -49,7 +49,7 @@ The view is not executed when it is defined. When the second step reads it, the
 
 ## What it looks like
 
-The demo table below has four rows. `LINE_VALUE` is `QTY * PRICE`, `REGION` is derived from `STATE`, and `REGION_LABEL` is built on `REGION`. None of the three is a column of the table, and all three are read-only in the grid:
+The demo table below has four rows. `LINE_VALUE` is `QTY * PRICE`, `REGION` is derived from `STATE`, and `REGION_LABEL` is built on `REGION`. None of the three is a column of the table, and all three are read-only in the grid - and marked so, with a tinted header carrying an `fx` badge, italic tinted cells, and an edge where the computed block begins, so that "computed" never has to be confused with "locked by a rule":
 
 ![](./derived-grid.png)