Fonts: missing fonts are listed when a document opens, with Find Fonts and a folder search

When a document opens with fonts that aren't available, fonts used
only in its symbols included, VectorCraft lists them in a Missing
Fonts dialog after the missing linked file questions, one document at
a time, with Find Fonts and Close. Find Fonts opens Type › Find Font
on the first missing font of the layers' type, to replace it. After
Find Fonts or Close the dialog doesn't open again for that open; when
another dialog replaces it, it opens again once that dialog closes.

Search Fonts Folder (the Fonts folder File › Package writes next to
the document) and Find in Folder… search a folder and its subfolders
for the fonts' files, and Add Fonts copies the files the user chooses
into a Fonts folder next to the preferences. The desktop app and
vectorcraft-cli read that folder with the system's font folders. Type
› Find Font has a Find in Folder… button too when fonts the document
uses are missing. The web app shows no dialog; a status message says
how many fonts aren't available.

- text.missingFonts lists each missing family and style once,
  whatever version the document's type names, and leaves out CSS
  generic families such as sans-serif. text.findFontFiles searches a
  folder on up to 8 threads and returns at once; text.findFontFiles {}
  reports the search, and {stop: true} stops it. A search reads only
  what the font scan reads of font files (their table directories and
  name, fvar and OS/2 tables) and, on macOS, of suitcase fonts, and
  lists a file for a font only when the file makes the font resolve
  exactly. It does not enter app packages, the Program Files and
  ProgramData folders, the system's folders at the root of a volume,
  the Shared and Public folders in Users, a home folder's Library,
  AppData, Applications, snap and dot folders, a Library folder's app
  data folders or the folders the environment names for apps, and it
  skips other hidden folders and does not follow links below the
  picked folder. It ends after 60 s by default, after 10,000,000
  entries listed, with more than 1,000,000 folders waiting, after
  10,000 font files read or 1,000 kept, or once every font has a file;
  Stop and Close end it too.
- text.addFontFiles copies only files the last search found. Of
  those, it copies only regular .ttf, .otf, .ttc, .otc, .woff and
  .woff2 files and suitcase fonts with their resource fork, at most
  256 MB a file and 1 GB in all. It never replaces a file: a file with
  the same contents is kept, and when another file has that name, the
  copy gets a number (Name 2.otf). The fonts are then scanned again,
  and type set in them redraws.
- ui.missingFontsDialog and ui.findFontsInFolder open the dialog for
  agents; docs/control-protocol.md lists its fields.
- File › Package gives a font file whose name has no font extension
  the extension that matches its data (.ttf, .otf or .ttc).
- vectorcraft-text adds set_app_font_dir, which system_font_dirs
  reads, WantedFonts, which reads a font file's names as the font scan
  does, is_font_file and is_suitcase. #762 split its face reader, which
  now takes read budgets: file_face_names_within tries the suitcase
  reader first, and reader_face_names_within applies the budgets to
  each font. vectorcraft-format adds write_new_with.

Docs: docs/mcp.md, docs/control-protocol.md, ROADMAP.

docs/control-protocol.md

@@ -375,6 +375,27 @@ folder; `ui.dialog.set {field: "discard", value: true}` then confirm ignores it
 Then, with the preference `updateLinks: "askWhenModified"`, modified linked files are offered for update in a
 `confirm` dialog whose `ui.dialog.confirm` runs `links.update`.
 
+Missing fonts: once the missing linked file questions are answered, or at once when there are none, `app.open` of a
+document whose type uses fonts that aren't available (`text.missingFonts`) opens the `missingFonts` dialog, one document
+at a time, and so does `links.editOriginal` of a placed document. When another dialog takes its place (Import PDF for a
+file opened next, for example), the dialog opens again once that one closes. Fields: `document` (the document's uid),
+`fonts` (`[{family, style, status, resolved}]`), `fontsNextToDocument` (the `Fonts` folder next to the document, or
+empty), `state` (empty, `searching`, `done`, `stopped` or `failed`), `search` (the search's `id`), `folder`, `found`
+(`text.findFontFiles`'s `fonts`, with the files found), `searched`, `skipped`, `unreadable`, `stopped`, `error` and
+`chosen` (the files to add; a font's first file is chosen when the search finds it). Its buttons are Find Fonts and
+Close, with Search Fonts Folder, Find in Folder… and, once files are found, Add Fonts in the dialog.
+`ui.dialog.set {field: "discard", value: true}` then `ui.dialog.confirm` is Find Fonts: the dialog closes and Type ›
+Find Font opens (dialog `findFont`) with the first missing font selected. `ui.dialog.set {field: "searchFolder",
+value}` then `ui.dialog.confirm` searches that folder (`text.findFontFiles`), keeps the dialog open and clears
+`searchFolder`, also when the search doesn't start. The fields change as frames draw the dialog; `text.findFontFiles
+{}` reports the search directly. `ui.dialog.confirm` without `discard` or `searchFolder` is Add Fonts: it copies the
+files in `chosen` (`text.addFontFiles`) and closes the dialog. `ui.dialog.cancel` (Close) closes it and stops its
+search. After Find Fonts or Close, the dialog doesn't open again for that open.
+`ui.missingFontsDialog {folder?}` opens the dialog for the active document, searching `folder` when given.
+`ui.findFontsInFolder {folder?}` (Find in Folder… in Type › Find Font…, shown when the document misses fonts) asks for a
+folder, then opens the dialog searching it. The web shows a status line instead of the dialog, and both commands are
+disabled there.
+
 Text Import Options: placing a `.txt` file through the app (`file.place` with a file and no `text` options, the Place
 dialog, a drop) opens the `textImport` dialog (fields `platform`: `windows`/`mac`, `characterSet`: `unicode`/`ansi`,
 `removeLineReturns`, `removeParagraphReturns`, `replaceSpaces`, `spaces`). `ui.dialog.confirm` places the file as area