Add the manual: every feature pictured from the application itself

docs/manual/README.md is a tour for a photographer opening DarkRoom for
the first time — one picture per thing, moving where movement is the
point. tools/manual/ is how the pictures are made: drive.py puppeteers the
desktop build on a private Xvfb (launch, click, drag, type, screenshot,
record), scenes.py is each picture as a script, and record.sh runs them
all over a folder and writes the results into docs/manual/media/.

The media is in LFS, with the CI pulls excluding it as they exclude the
fixtures; a screenshot changes wholesale when the interface does.

Nothing in the pictures shows a person, by design: the demo library is
seventy urban and alpine frames, chosen from the catalog's rows that face
detection found nobody in.

The traceability matrix is regenerated here after the rebase that
brought this branch up to master.

docs/manual/README.md

@@ -0,0 +1,224 @@
+# DarkRoom, shown
+
+A tour of what the application does, one picture per thing. Every image on
+this page was captured from the desktop build driving itself — nothing is a
+mock-up, and nothing has been retouched outside DarkRoom. Where a feature is
+better seen moving, it moves.
+
+The requirements behind each feature are in [requirements.md](../requirements.md);
+the reasoning is in the design documents linked from each section. This page
+is only about what you see.
+
+The photographs are the author's. None show a person.
+
+## Opening a library
+
+DarkRoom opens on a library: a folder on this machine, a folder a sync
+client keeps, or a Nextcloud account. A folder needs no password and uploads
+nothing.
+
+![The launch screen: a server field, a folder field, and which formats to scan for](media/launch.png)
+
+Once a folder is named, it is the library — you are not asked for it again,
+and `Open library` opens it whole. `Subfolder…` narrows the scan to part of
+it. The formats ticked are what the scan looks for; RAW is on and JPEG off
+by default, because a RAW editor's sensible default is the file the camera
+wrote first.
+
+![A folder chosen: the library, whether to scan a subfolder, and the formats](media/launch-folder.png)
+
+## The library
+
+![The grid: collections on the left, the timeline beside it, the roll of thumbnails, and the filter bar above](media/library.png)
+
+Photographs are ordered by capture time, with a month heading where each
+begins. The strip on the left is the timeline — drag it to jump to a year.
+The bar above the grid filters by rating, flag and where the file is (on this
+device, or only on the server).
+
+### Rating and flagging
+
+Hover a cell and the stars appear; click one. The filter chips above the grid
+count what each rating holds, and clicking `3+` shows only those.
+
+![Rating two photographs, then filtering the grid to three stars and more](media/library-rating.gif)
+
+### Getting about
+
+Drag the timeline to scrub through years; Ctrl and the wheel resize the
+thumbnails.
+
+![Scrubbing the timeline](media/library-timeline.gif)
+
+![Resizing the thumbnails with Ctrl and the wheel](media/library-thumbsize.gif)
+
+### Selecting several
+
+`Select` in the header — or Ctrl-click — starts a selection. Shift-click picks
+a range. The bar at the foot of the grid is everything a selection can be done
+to: collections, keywords, presets, export, and merging to a panorama.
+
+![Twelve photographs selected, with the selection bar along the foot of the grid](media/library-selection.png)
+
+`Keywords` on that bar opens a sheet; type a word and press return, and it
+is on every photograph selected. The list below the field is every keyword
+the library has, ticked where the selection carries it.
+
+![Keywording twelve frames](media/library-keywords.gif)
+
+### Collections
+
+`+` at the head of the sidebar makes one. Drag a photograph — or the whole
+selection — onto its row to file it there; click the row to see it. A
+photograph can be in several, and the badge on its cell counts them.
+
+![Filing photographs in a collection by dragging them onto it](media/library-collections.gif)
+
+## Developing a photograph
+
+Click a thumbnail to open it. The column on the right is every adjustment;
+the strip at its head narrows it to one group.
+
+![The develop view: the photograph, the histogram, and the adjustment column](media/develop.png)
+
+![Switching between the Optics, Light, Colour, Effects and Detail groups](media/develop-groups.gif)
+
+### Light
+
+Exposure, contrast, highlights, shadows, blacks, whites and a tone curve.
+Hold `Before` to see the photograph as it was.
+
+![Raising exposure, pulling the highlights, lifting the shadows, then holding Before](media/develop-light.gif)
+
+### Looking closer
+
+Double-click for 1:1; drag to move about; double-click again to fit. The
+wheel zooms to any amount in between.
+
+![Zooming to 1:1, panning, and back](media/develop-zoom.gif)
+
+### White balance from the photograph
+
+Press `pick` in the White Balance group, then click something neutral.
+
+![Picking a neutral wall to set the white balance](media/develop-wb.gif)
+
+### Composing
+
+Crop by dragging the frame's corners, straighten with the slider, lock a
+ratio from the chips. `Done composing` returns to the photograph.
+
+![Cropping, straightening and choosing a ratio](media/compose.gif)
+
+### Local adjustments
+
+`Local` in the rail turns the column into a mask stack. `Find subjects` runs a
+segmentation model over the photograph; what it recognises appears as a list
+of categories with how much of the frame each covers. Click one and it is a
+mask — then every slider below edits only that region.
+
+![What the model found in an urban scene: ground, architecture, sky, vegetation](media/local-categories.png)
+
+![The sky chosen: tinted on the photograph, and the column now scoped to it](media/local-segment.png)
+
+A mask is a stack of parts. Paint into it, subtract a gradient from it, grow
+or shrink its edge, choose how it falls off. `Show masks as` draws the mask
+tinted, as alpha, or as an outline; the eye on its row switches it off.
+
+![Looking at the mask three ways, painting into it, then growing its edge](media/local-paint.gif)
+
+Linear and radial gradients, a tone range and a colour range are the other
+ways to make one; each can be combined with any other.
+
+### Repair
+
+`Repair` in the rail: click a mark and it is covered from a source DarkRoom
+chooses beside it. Drag either circle to move it; the size, feather and
+opacity are in the panel. Heal blends; clone copies.
+
+![Covering marks on a road](media/repair.gif)
+
+### Film
+
+The `Film` chooser at the head of Adjust applies a spectral simulation of a
+named stock; below it, the print exposure and push controls a film has and a
+sensor does not.
+
+![Choosing Velvia, then holding Before](media/film.gif)
+
+### History, snapshots, presets
+
+Every change is a step; `Undo` and the History panel walk them. `Snapshot`
+keeps the current state under a name. `Presets…` saves the settings to
+apply elsewhere, and imports `.xmp` from other applications.
+
+![The presets sheet](media/presets.png)
+
+## Merging a panorama
+
+Select the frames, then `Merge to panorama` from the selection bar. The
+frames are read, aligned, and drawn on the suggested projection with each one
+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.
+
+![Twelve frames aligned, the projections tried, and the border filled](media/panorama.gif)
+
+![The alignment on a cylinder, each frame outlined where it landed](media/panorama-aligned.png)
+
+![The same, with the ragged border filled by the model rather than cropped away](media/panorama-filled.png)
+
+## Export
+
+`Export` in the develop header, or `Export N` from a selection. Format,
+size, colour space, sharpening, naming and where the file goes are in
+Settings, and apply to every export until changed. An export with no folder
+set is refused, and the header says so.
+
+![Export defaults in Settings](media/settings-export.png)
+
+## Settings
+
+![The settings page: background activity, indexing, storage, display](media/settings.png)
+
+Background activity with progress, thumbnail and face indexing, storage on
+this device, display and colour, export defaults, what is written to XMP
+sidecars, and a diagnostics bundle for a bug report.
+
+## People
+
+Face detection and identity run over the library and group faces by person;
+the `Identity` page is where suggestions are confirmed, rejected and split,
+and `People` on the filter bar narrows the grid to someone. Not pictured
+here, for the obvious reason — [faces.md](../faces.md) has the design.
+
+## Where things are written down
+
+| Feature | Design |
+|---|---|
+| Local masks and segmentation | [segmentation.md](../segmentation.md), [mask-editing.md](../mask-editing.md) |
+| Repair | [spot-removal.md](../spot-removal.md) |
+| Panorama | [panorama.md](../panorama.md) |
+| Faces and identity | [faces.md](../faces.md) |
+| Gestures, generated from the code | [gestures.md](../gestures.md) |
+| Navigation and layout | [ui-navigation.md](../ui-navigation.md) |
+| Sync and storage | [storage.md](../storage.md) |
+
+## How this page is made
+
+[`tools/manual/`](../../tools/manual/README.md) drives the desktop build on
+a private X server and records each scene; `record.sh <library>` re-makes
+every picture here. Run it after a change to the interface and commit what
+changed. The pictures are in LFS.
+
+Making it the first time turned up nine faults, each fixed in its own
+commit before the pictures were taken: the folder picker could not choose
+the top level, month headings overprinted each other, a category mask
+widened the column off the window, the mask tint outlived its mode, the
+first sync uploaded an empty thumbnail shard, a merge that ran out of GPU
+memory left the page on Stop for ever, the export settings promised to ask
+for a folder and did not, the keyword sheet sent its keys to the grid, and
+an empty trash told you to check your library folder.