The desktop app links SMLib by default

SMLib is the only kernel, so a desktop build without it models nothing. The app gets the
smlib feature the CLI has, on by default.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HoZgkCdAVrscuw2kjQTWDs

apps/solidcraft/Cargo.toml

@@ -8,9 +8,12 @@ publish = false
 default-run = "solidcraft"
 
 [features]
-default = ["raytrace-cpu", "bundled-render-library"]
+default = ["raytrace-cpu", "bundled-render-library", "smlib"]
 bundled-render-library = ["solidcraft-engine/bundled-render-library"]
 raytrace-cpu = ["solidcraft-engine/raytrace-cpu"]
+# Link SMLib into the kernel (solidcraft-engine/smlib; needs scripts/fetch-smlib.sh). SMLib is the
+# only kernel, so without it the app models nothing.
+smlib = ["solidcraft-engine/smlib"]
 
 # The workspace lints, except unsafe_code: denied, not forbidden, so that src/open_documents.rs
 # (macOS file-open events), src/native_crash.rs (Windows exception filter), src/native_crash_macos.rs