Drops: libraries, presets and plug-ins open on a drop, as File › Open opens them

With a document open, the desktop app ran File › Place on a swatch or
graphic style library, a Libraries panel file, a presets file or a
plug-in dropped on it. A desktop drop has no position, and without one
the app opened only documents and ran File › Place on every other file.
On the web, the app also ran File › Place on these files when they were
dropped on the canvas. File › Place can't read them, and the drop
failed with "Couldn't place X: ... not a format VectorCraft reads".

VectorcraftApp::drop_target now returns Open for these files, with or
without a drop position, and they open as File › Open opens them:
plug-ins are installed, presets are imported, swatch and graphic style
libraries open in the library panel, and a Libraries panel file is
added to the Libraries panel as a new library. A dropped library,
presets file or plug-in that has a path is added to Open Recent Files
when it opens, as File › Open adds it. io::opens_as returns the kind of
each of these files from its extension, and drop_target and
io::open_bytes both call it. The rules from #472 still apply to
documents, pictures and text. On the web the browser gives no file name
before the drop, so the app still outlines the canvas while such a file
is dragged over it.

Tests: drop_target returns Open for each of the eleven extensions, with
no position and with a position on the canvas. A .gpl dropped through
the desktop drop path loads in the library panel and is added to Open
Recent Files, and a .vclibrary dropped the same way becomes a new
library in the Libraries panel.

Docs: docs/control-protocol.md (file.place), docs/development.md
(Drag-and-drop), docs/target-app-parity.md (Evidence, UI).

docs/control-protocol.md

@@ -69,7 +69,7 @@ the two can still win that race.
 | `engine.execute` `ui.savePdfDialog` | `{path?, preset?, range?, …document.exportPdf options}` | opens the Save PDF dialog (`file.export.pdf` with no params opens it too). Its fields are the `document.exportPdf` options: set `compatibility` with `ui.dialog.set {field: "compatibility", value: "1.5"}`, a whole section with `{field: "compression", value: {…}}`, the artboards with `__allArtboards: false` and `range`, the visible section with `__section` (General, Compression, Marks and Bleeds, Output, Advanced, Security, Summary). `ui.dialog.confirm` writes `path` (else asks for one), returns `{path, bytes, warnings}` and opens the file when `viewAfterSaving` is set |
 | `file.save` / `file.saveAs` / `file.saveCopy` / `file.saveAsTemplate` (via `engine.execute`) | `{path?, format?, options?, svg?}` | Save writes the document's own file in its own format (a document opened from or saved as SVG/PDF saves as that again; the result's `warnings` say what the format loses). With `path` the file is written at once (Save As or a Copy to an SVG path without SVG options first opens SVG Options, below). Without one (never saved, a converted older file, Save As, a Copy, a Template) the app shows a save panel with one file type per save format (`vectorcraft`, `template`, `pdf`, `svg`, `svgz`), then the format's options: SVG opens the `svgOptions` dialog (below; returns `{"pending": "svgOptions", "path"}`), PDF the `savePdf` dialog (its `__save` field names the save command); `ui.dialog.confirm` finishes the save. On the web a `saveOptions` dialog (fields `path`, `format`) first names the file and picks the format. `file.formatOptions {format?}` reads the options a save would use. `file.revert` opens a `confirm` dialog (`{"pending": "confirm"}`); `ui.dialog.confirm` reloads the saved file in the same tab |
 | SVG Options (`engine.execute` of `file.export.svg`, `file.saveAs`, `file.saveCopy`) | `{}` / `{path?, svg?: {…}}` | `file.export.svg` without params, and Save As or Save a Copy to an `.svg`/`.svgz` path without SVG options, open the `svgOptions` dialog (`ui.dialog.set` its fields: `styling`, `outlineText`, `images`, `objectIds`, `decimals`, `minify`, `responsive`, `fewerTspans`, `metadata`, `preserveEditing`, `useArtboards`, `allArtboards`, `range`, `showCode`, `profile` (`svg11`/`tiny12`), `encoding` (`utf8`/`utf16`/`latin1`), `embedFonts`; `ui.dialog.confirm` exports or saves and remembers the choices). With `svg: {…}` (the `document.formats` SVG options) they run directly. `app.save {path?, svg?}` takes the same options; a document saved as SVG saves as SVG again with the same options |
-| `engine.execute` `file.place` | `{paths?}` / `{path \| name+dataBase64, …file.place params}` | with no file, picks files and opens the `place` dialog; with `paths`, opens it for them (fields `files`, `link`, `template`, `replace`; `__replace` says whether Replace applies, `__info` describes each file). `ui.dialog.confirm` places one file centred in the view (or in place of the selected object) and loads the place cursor with several (`file.place.queue`). With a file, it runs `file.place` centred in the view. Files dropped on the window with no document open open. On the web, files dropped on the canvas are placed at the drop point (linked; Shift embeds) and files dropped off it (the tab bar) open, as in Illustrator; desktop drops carry no pointer position, so documents (native, SVG, PDF, `.ai`, EPS, DXF, EMF/WMF) open as tabs of their own and pictures and text are placed in the middle of the view. With the cursor loaded (`ui.inspect` → `tool: "place"`, `toolOptions: {count, current, name, names}`), `ui.pointer` clicks and drags place files and `ui.key` ←/→/↑/↓ and Escape cycle and discard them |
+| `engine.execute` `file.place` | `{paths?}` / `{path \| name+dataBase64, …file.place params}` | with no file, picks files and opens the `place` dialog; with `paths`, opens it for them (fields `files`, `link`, `template`, `replace`; `__replace` says whether Replace applies, `__info` describes each file). `ui.dialog.confirm` places one file centred in the view (or in place of the selected object) and loads the place cursor with several (`file.place.queue`). With a file, it runs `file.place` centred in the view. Files dropped on the window with no document open open. On the web, files dropped on the canvas are placed at the drop point (linked; Shift embeds) and files dropped off it (the tab bar) open, as in Illustrator; desktop drops carry no pointer position, so documents (native, SVG, PDF, `.ai`, EPS, DXF, EMF/WMF) open as tabs of their own and pictures and text are placed in the middle of the view. Swatch libraries and color books (`.vcswatches`, `.gpl`, `.ase`, `.acb`), graphic style libraries (`.vcstyles`), Libraries panel files (`.vclibrary`), presets (`.vcflattener`, `.vcpdfpresets`, `.vcprintpresets`, `.vcperspective`) and plug-ins (`.wasm`) open wherever they are dropped, as File › Open opens them. With the cursor loaded (`ui.inspect` → `tool: "place"`, `toolOptions: {count, current, name, names}`), `ui.pointer` clicks and drags place files and `ui.key` ←/→/↑/↓ and Escape cycle and discard them |
 | `app.export` raster options | `{path?, format: png \| jpg \| webp, ppi?, background?, antiAlias?, interlaced?, quality?}` | `ppi` sets the pixel size (72 = one pixel per point) and is stored in the file; `background` is `transparent`, `white`, `black` or `"#rrggbb"`; `antiAlias` is `none`, `art` or `type` (text snapped to pixels); `interlaced` writes an Adam7 PNG. The PNG Options dialog (`pngOptions`; `jpgOptions` / `webpOptions` for the other raster formats) has the same fields: fill them with `ui.dialog.set` and press OK with `ui.dialog.confirm` |
 | `app.export` JPEG options | `{path?, format: jpg, quality?: 0–100, colorModel?: rgb \| cmyk \| gray, method?: baseline \| optimized \| progressive, scans?: 3–5, embedIcc?, imageMap?: none \| client \| server}` | `cmyk` writes ink amounts in the working CMYK space (CMYK colours keep their inks; RGB ones are separated with the colour settings); `embedIcc` (default true) embeds sRGB, the working CMYK profile or a grey profile, generated by the CMS; `imageMap` also writes `<stem>.html` (client-side `<map>`) or `<stem>.map` (NCSA) beside the image for the objects with a URL and an Image Map shape (`attributes.set`), returned in `linked`. The `jpgOptions` dialog has these fields too (`quality` is shown 0–10; a CMYK document defaults to `cmyk`) |
 | `app.export` PNG-8 and GIF | `{path?, format: png8 \| gif, colors?: 2–256, reduction?: perceptual \| selective \| adaptive \| web \| blackWhite \| gray, dither?: none \| diffusion \| pattern \| noise, ditherAmount?: 0–100, transparency?, matte?, interlaced?, …raster options}` | a palette of at most `colors` entries (art with that many colours or fewer keeps them exactly); pixels under half opacity become one transparent entry unless `transparency` is false, partly transparent edges are blended over `matte` (`none` keeps their colour). `png8` writes an indexed `.png` (1–8 bits per pixel); Export for Screens rows take both. The `png8Options` / `gifOptions` dialogs have these fields |