Text: a bundled-fonts feature (on by default) lets embedders that bring their own fonts leave ours out

The nine OFL faces in `assets/fonts` are compiled into every build of
vectorcraft-text: 3.4 MB of the binary, about 1.6 MB of a gzipped wasm.
An embedder that loads every font at run time with `FontDb::add_font`
(a web app serving its own font catalogue, say) ships them twice and
can't turn them off.

- vectorcraft-text: `bundled-fonts`, a default feature, holds the
  `include_bytes!` list. Off, a database starts without them (craft-fonts
  and system fonts are unaffected) and text has no glyphs until a font
  is added, as when the bundled fonts fail to parse.
- vectorcraft-engine: a `bundled-fonts` default feature forwards to it,
  so the apps and any embedder using the engine keep the fonts unless
  they build it with `default-features = false`.
- The workspace declares vectorcraft-text without default features, so
  no library crate turns the fonts back on. The tests of cad, metafile
  and tools, which lay type out, ask for the feature in their
  dev-dependencies; the other crates' tests get it through testkit.

`cargo test --workspace` builds with the fonts, as before. A new
integration test, `without_bundled_fonts`, covers the configuration
without them (`cargo test -p vectorcraft-text --no-default-features
--test without_bundled_fonts`): the database starts without them, text
in no font lays out with no glyphs, and a font added later is used and
is the fallback for families the database lacks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/Cargo.toml

@@ -8,6 +8,12 @@ description = "VectorCraft engine: sessions, history, selection, the command reg
 [lints]
 workspace = true
 
+[features]
+default = ["bundled-fonts"]
+# The fonts vectorcraft-text bundles (see its `bundled-fonts`). Embedders that supply every font at
+# run time (`vectorcraft_text::FontDb::add_font`) build with `default-features = false`.
+bundled-fonts = ["vectorcraft-text/bundled-fonts"]
+
 [dependencies]
 vectorcraft-geom = { workspace = true }
 vectorcraft-color = { workspace = true }