Fix #306: render on the power-saving GPU by default, with a preference for the discrete one

On hybrid-graphics laptops (an integrated GPU driving the display plus a
discrete one), egui-wgpu's default power preference is HighPerformance, so
VectorCraft rendered on the discrete GPU and its frames were copied to the
display through the integrated one. On an Asus ZenBook with Intel Iris + Arc
the window flickered whenever the mouse moved over it (#306, #305).

The canvas is rasterized on the CPU (vello_cpu) and the GPU only composites
the canvas texture and draws the UI, so the integrated GPU is plenty. The
desktop app now asks wgpu for the power-saving adapter by default.
Single-GPU machines are unaffected.

- Preferences > Performance > Graphics Processor (`gpuPreference`:
  powerSaving, the default, or highPerformance), with a note that it
  applies after a restart. The app reads it from the saved preferences
  before the window opens. WGPU_POWER_PREF still overrides it.
- Help > About and `ui.inspect` (`graphicsAdapter`) name the adapter in
  use, and it is logged at startup, for future GPU bug reports.
- Docs (README, docs/development.md) and ROADMAP updated.

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

crates/engine/src/cmd/prefscmds.rs

@@ -82,6 +82,8 @@ pub const UNITS: &[(&str, &str)] = &[
     ("feet", "Feet"),
 ];
 const LINE_STYLE: &[(&str, &str)] = &[("lines", "Lines"), ("dots", "Dots")];
+/// Performance › Graphics Processor (`gpuPreference`), read by the desktop app at startup.
+pub const GPU_PREFERENCES: &[(&str, &str)] = &[("powerSaving", "Power Saving (integrated)"), ("highPerformance", "High Performance (discrete)")];
 const BLACK: &[(&str, &str)] = &[("accurate", "Display All Blacks Accurately"), ("rich", "Display All Blacks as Rich Black")];
 const BLACK_OUT: &[(&str, &str)] = &[("accurate", "Output All Blacks Accurately"), ("rich", "Output All Blacks as Rich Black")];
 
@@ -265,6 +267,7 @@ pub const PREF_SPECS: &[PrefSpec] = &[
     // Performance
     p!("gpuPerformance", "Performance", "GPU Performance", "GPU Performance", bool),
     p!("animatedZoom", "Performance", "GPU Performance", "Animated Zoom", bool),
+    p!("gpuPreference", "Performance", "GPU Performance", "Graphics Processor", choice(GPU_PREFERENCES)),
     p!("historyStates", "Performance", "", "History States", int(5, 1000)),
     p!("realTimeDrawing", "Performance", "", "Real-time Drawing and Editing", bool),
     p!("renderThreads", "Performance", "", "Render Threads (-1 = Automatic)", int(-1, 64)),