Fonts: a removed font that type used leaves the list at the next font scan

When type had used a font, removing the font while the app ran left
it in the font menus, and the type kept drawing in it until the app
restarted. The font list is the loaded faces plus the scanned catalog,
and a scan replaced the catalog but kept every loaded face.

A scan now drops each loaded face whose file it did not catalog, with
the face's cached outlines. A family with no face left leaves the font
lists, and type set in it lays out in the fallback font and is
reported missing, as when a document opens without the font. When the
font is installed again, the next scan lists it and the type draws in
it again. Bundled fonts and fonts added as bytes have no file and
stay.

Docs: docs/mcp.md, docs/target-app-parity.md.

crates/engine/src/cmd/fonts.rs

@@ -56,7 +56,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Refresh Font List",
             [],
             None,
-            "{} scan the system font folders again, for fonts installed or removed since the app started; text in a font that became available redraws in it (no fonts are installed on the web) → {families, faces} (families listed, installed faces found)",
+            "{} scan the system font folders again, for fonts installed or removed since the app started; text in a font that became available redraws in it, and text in a family that was removed redraws in the fallback font (no fonts are installed on the web) → {families, faces} (families listed, installed faces found)",
             always,
             rescan
         ),