Show the composite in the grid in the manual, and say how it gets there

The panorama scene now ends in the grid, on the composite in its wide
cell beside the twelve frames it was made from, with the thumbnail the
merge made: the picture is taken straight after the merge, before any
rescan could have found the file. The manual's library section says how
a panorama's cell is sized and packed, and its panorama section that the
composite is in the grid the moment it is written. panorama.md §9 records
the race that kept it out, the catalogue-then-register path that
replaced it, how the thumbnail is developed, and the layout's classes.

docs/manual/README.md

@@ -75,6 +75,14 @@ past the window. On a tablet they scroll by flick alone, and a thin line at
 the right-hand edge shows where the view is while it moves, fading once it
 stops; it is only a picture, and a flick that starts on it scrolls the list.
 
+A panorama gets a wider cell: about twice as wide as it is tall and it spans
+two columns, then three, then four for the widest — whichever leaves the least
+of the cell empty — with a thumbnail made for that width. One that would not
+fit in what is left of a row starts the next, so the grid still reads in the
+order the photographs were taken; the arrows walk it in that order, and up and
+down go to whatever is above or below. On a tablet, or with too few columns to
+put it beside anything, a panorama takes the whole row.
+
 `Help` in the header, or `F1`, opens the controls and shortcuts: every key and
 gesture, screen by screen, with `See it` beside those this page shows, and
 `Manual` to open this page. In develop it is the `?` beside `Settings`.
@@ -345,8 +353,13 @@ outlined where it landed — twelve hand-held portrait frames across an alpine
 valley, here. Change the projection (a 150° sweep on a flat perspective is
 what the middle of the film shows, and why cylindrical is suggested), ask for
 the border to be filled rather than cropped, then `Merge`. The composite is
-written beside its sources as a DNG and appears in the grid with the merge
-as the first step in its history.
+written beside its sources as a DNG and is in the grid the moment it is
+written — in a wide cell beside its frames, placed by when they were taken —
+with the merge as the first step in its history. Its thumbnail is made during
+the merge, from the finished picture, as develop will show it when you open
+it; on a server library it is in the grid while the file is still uploading.
+A second merge of the same frames is named `-pano-2`, never written over the
+first.
 
 Each frame has a box in the `Frames` list. Untick one to leave it out, and
 the rest are aligned again at once, without reading the frames again; tick
@@ -367,6 +380,8 @@ goes for a panorama Lightroom stitched and saved as a DNG.
 
 ![The same, with the ragged border filled by the model rather than cropped away](media/panorama-filled.png)
 
+![The composite in the grid straight after the merge, spanning four columns beside the twelve frames it was made from](media/panorama-in-grid.png)
+
 ## Export
 
 `Export` in the develop header, or `Export N` from a selection. Format,