Export: transparent background docs and clearer refusal (#784)

The error for a non-PNG export says the option is for PNG export. ROADMAP,
file-format, UI parity and control protocol docs mention the option.

docs/control-protocol.md

@@ -45,7 +45,7 @@ Coordinates are slide points (1/72 inch; a 16:9 slide is 960 × 540) unless note
 | `ui.render` | `{path?, slide?, scale?}` | Render a slide headlessly |
 | `ui.resize` / `ui.focus` | | Window control |
 | `show.start` / `show.end` | `{from?: slide index, custom?: name, reading?}` | Slide show. `custom` plays that custom show from its first slide and does not change Set Up. With no `from` and no `custom`, playback starts at the first slide of the show order |
-| `app.open` / `app.save` / `app.export` / `app.quit` | `{path, …}` | Files (export takes the `file.export` params, e.g. `{"path": "deck.pdf", "layout": "handouts", "perPage": 3}` or `{"path": "deck.gif", "width": "large", "secondsPerSlide": 3, "loop": false}`) |
+| `app.open` / `app.save` / `app.export` / `app.quit` | `{path, …}` | Files (export takes the `file.export` params, e.g. `{"path": "deck.pdf", "layout": "handouts", "perPage": 3}` or `{"path": "deck.gif", "width": "large", "secondsPerSlide": 3, "loop": false}`, or `{"path": "slide.png", "transparentBackground": true}` for a PNG without the slide background) |
 
 The MCP server (`deckcraft-cli mcp`, see [mcp.md](mcp.md)) wraps the same methods for Claude
 and other agents; `deckcraft-cli app --method METHOD [PARAMS]` sends one request from a shell.

docs/file-format-parity.md

@@ -26,7 +26,7 @@ PowerPoint opens and saves; Viewer = opens only).
 | `.deckcraft` native (zip + JSON) | n/a | yes (also `.slidecraft`) | yes | Lossless for the DeckCraft model, forward compatible | `crates/format` tests |
 | PDF | export | no | yes | Slides, notes pages, handouts (1–9); real-text layer with embedded subsets, links, bookmarks. Slide artwork is a raster image at print resolution, not vector | `crates/pdf` tests |
 | Outline `.rtf` / `.txt` | export (RTF), import (outline) | `.txt` outline; `.md` as Markdown (headings are slides, list items bullets) (`slide.fromOutline`) | `.txt` outline | No RTF | engine tests |
-| PNG / JPEG slide images | export | n/a | yes (`file.export`, `deckcraft-cli render`) | Matches the editor's renderer. PNG `pHYs` and JPEG JFIF density are `scale` × 72 dpi (default scale 2 is 144 dpi), so Preview shows the slide's real size | render and engine tests |
+| PNG / JPEG slide images | export | n/a | yes (`file.export`, `deckcraft-cli render`) | Matches the editor's renderer. PNG `pHYs` and JPEG JFIF density are `scale` × 72 dpi (default scale 2 is 144 dpi), so Preview shows the slide's real size. PNG can leave the slide background out (`transparentBackground`, Export… “Transparent background”, `deckcraft-cli render --transparent`), so only the slide's content is opaque (#784) | render, engine, UI and CLI tests |
 | TIFF / BMP / GIF slide images | export | n/a | no | Easy (the `image` crate writes them) | none |
 | Animated GIF of the deck | export | n/a | yes (`file.export` format `gif`, Export…, `deckcraft-cli`, MCP `export`) | One frame per slide from the slide renderer; size small/medium/large/extra large or 16–3840 px wide, 0.1–600 s per slide, loop forever or play once, slide range, hidden slides skipped; a 256-colour palette per frame (exact, or NeuQuant above 256 colours). Transitions and build animations are not animated | `crates/engine/src/gif_export.rs` tests (decoded frames, size, delay, loop, hidden slides, hostile params), MCP, CLI and export dialog tests |
 | MP4 / MOV video of the deck | export | n/a | no | Needs timeline render + H.264 encode | none |
@@ -103,6 +103,7 @@ the round trip even though DeckCraft doesn't model it.
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-16 | minor | PNG slide export with a transparent background (#784) |
 | 2026-10-14 | minor | Community PR wave folded: one row each for `.pptx` (seven owned corpus files), outline, 3-D models, embedded fonts, text, effects, 3-D (extrusion sides draw now), SmartArt (written as a diagram) and VBA (tags and custom XML are kept) |
 | 2026-10-12 | minor | SmartArt import lays each shape's text out on its `dsp:txXfrm` text rectangle instead of the shape box |
 | 2026-10-13 | minor | EMF/WMF rectangle clip regions limit later drawing; EMF+ records remain |

docs/ui-parity.md

@@ -55,7 +55,7 @@ menu colours taken from the theme tokens (menu `chrome`, page `card`).
 | Save As | Places, folder, file name, type, Save, More options | Type (PowerPoint, DeckCraft, PDF, Outline, PNG, JPEG) and Save, which opens the system Save dialog (web: Download). .pptx/.deckcraft take the new file; the others are copies (`file.saveAs`, `file.export`) |
 | Print | Print and Copies, Printer, Print All / Selection / Current / Custom Range, Slides box, layout (Full Page Slides, Notes Pages, Outline, Handouts 1–9), sides, collation, colour, Edit Header & Footer; page preview with pager and zoom | Print, Print All Slides / Current Slide / Custom Range, Slides box (`1-3, 5`), Full Page Slides / Notes Pages / Handouts 1–9, Print Hidden Slides, Frame Slides, Edit Header & Footer; preview with pager. DeckCraft prints through a PDF (`file.export`) opened in the system viewer, which has the printers (web: downloaded). No printer list, copies, sides or colour |
 | Share | Cloud sharing | Not offered |
-| Export | Create PDF/XPS Document, Create a Video, Create an Animated GIF, Package Presentation for CD, Create Handouts, Change File Type | Create PDF Document, Create Handouts (a PDF), Change File Type. No video, GIF or CD package |
+| Export | Create PDF/XPS Document, Create a Video, Create an Animated GIF, Package Presentation for CD, Create Handouts, Change File Type | Create PDF Document, Create Handouts (a PDF), Change File Type (PNG with an optional transparent background, #784). No video, GIF or CD package |
 | Close | Closes; asks Save / Don't Save / Cancel about unsaved changes | Same (`file.close` after the prompt); Ctrl+W asks too |
 | Account | User information, sign-in, Office Background and Office Theme, product information, About | Appearance (light/dark, `view.dark`), product information, About DeckCraft. No accounts to sign in to |
 | Options | Opens the PowerPoint Options dialog | Opens Preferences |
@@ -165,6 +165,7 @@ consistency is open (#73, #56).
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-16 | minor | Export › Change File Type › PNG and the Export… dialog offer a transparent background (#784) |
 | 2026-10-14 | minor | Gray list markers on empty paragraphs while editing; caret keeps the list indent before typing |
 | 2026-10-14 | minor | Community PR wave folded: one row each for Format Shape, Dialogs (41, listed from `dialogs.rs`), Animation Pane (timeline is in), window, Quick Access Toolbar, Format Background, rulers, image paste, context menus, exact size and edit points; estimates re-totalled (~26–46 h) |
 | 2026-10-13 | trivial | Eyedropper in the colour menus (#367) |