Compile dr-ui on sixteen codegen units in release builds

Slint expands the .slint files into ~27 MB of Rust, and at the workspace's
single codegen unit LLVM optimised all of it on one thread: 13.5 minutes
of a release build with the other cores idle. The override applies to
dr-ui alone; the image crates keep one unit, and thin LTO still runs at
link time.

Cargo.toml

@@ -278,6 +278,14 @@ opt-level = 0
 lto = "thin"
 codegen-units = 1
 
+# Except dr-ui. Slint expands the `.slint` files into ~27 MB of Rust
+# (`out/app.rs`), and at one codegen unit LLVM optimises all of it on a single
+# thread: 13.5 minutes of a release build with the other cores idle. The code
+# it holds is UI glue — property bindings and callbacks — not the image work,
+# which lives in the crates above that keep the single unit.
+[profile.release.package.dr-ui]
+codegen-units = 16
+
 # A release build that can say where it panicked: line tables, so a crash
 # record's backtrace (`dr_plat::crash`) reads `file.rs:123` rather than bare
 # addresses. The macOS build uses it (docs/dev/macos.md) — no one here can