LRC-SS-LAYOUT: slideshow layout, options and backdrop: slides composed with margins, stroke, shadow and backdrop

- New crate lightcraft-slideshow (L1, no UI): SlideSettings (margins linked or
  independent, guides, aspect preview, zoom to fill, stroke border, cast shadow,
  backdrop colour, colour wash, background image; lengths relative to the slide's
  shorter side; validated and clamped) and a CPU compose() used by playback, the
  Settings preview and slideshow.preview.
- Engine: slideshow.layout (partial merge, saved in prefs.json) and
  slideshow.preview {id?, width?, height?, path?} (PNG to path or base64);
  RenderJob::with_slide composes on the render worker; the background photo is
  rendered once and shared.
- Playback: with a non-default layout each slide is the composed screen-sized
  slide, prepared ahead like before; the default layout plays as before.
- Settings > Slideshow: Layout, Options and Backdrop sections and a debounced
  live preview of the active photo's slide (with margin guides).
- Tests: composer geometry/pixels/hostile input, command get/set/persist/preview,
  headless playback of a composed slide and of the default layout.

docs/mcp.md

@@ -196,6 +196,12 @@ JSON params (documented in each tool's description and by `list_commands`). Pass
 leave these per-command tools out when a client struggles with a large tool list; `run_command`
 still reaches every command.
 
+Slideshow slides, for example: `cmd_slideshow_layout` reads or changes the slide layout (margins,
+zoom to fill, stroke border, cast shadow, backdrop colour, colour wash, background image; saved with
+the library) and `cmd_slideshow_preview {id?, width?, height?, path?}` composes one slide with it —
+written as a PNG to `path`, or returned as base64 PNG in `png` — so an agent can look at a layout
+before playing it. `cmd_slideshow_playback` sets the timing and fades.
+
 ## Resources
 
 `resources/list` / `resources/read` serve JSON snapshots: