Add desktop widgets and streamline documentation

docs/nested-resize-testing.md

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-# Nested live-resize regression check
-
-The shell enables libcosmic's `wgpu` feature. Without it, iced uses tiny-skia
-to resample and rasterize the wallpaper on the CPU. In an unoptimized build,
-this blocks the UI/render thread shared with the top bar for seconds. Prompt
-layer configures, buffer stretching, and compositor vsync cannot fix that
-client-side bottleneck. Software rendering remains available as a fallback.
-
-## Reproduce and compare
-
-```sh
-cargo build --locked -p ferese -p ferese-shell
-WAYLAND_DEBUG=1 FERESE_TRACE_PERFORMANCE=1 \
-  target/debug/ferese --backend nested --grant-effects --grant-shell-control -- \
-  target/debug/ferese-shell 2>/tmp/ferese-resize.log
-```
-
-Use a configured wallpaper. Drag the host window's edges in both directions,
-then maximize/restore it repeatedly. Check that the wallpaper and bar follow
-the window, including at fractional host scaling. Repeat with
-`ICED_BACKEND=wgpu` to require GPU rendering (rather than silently accepting
-fallback), and with `ICED_BACKEND=tiny-skia` to compare software rendering.
-Use a release build if software fallback is needed for regular use.
-
-Trace the chain, not just the compositor's frame rate:
-
-1. Host `xdg_toplevel.configure` becomes Winit `Resized`.
-2. Ferese updates the output, arranges layers, and sends
-   `zwlr_layer_surface_v1.configure` for the new logical dimensions.
-3. The shell acknowledges configure and eventually attaches/commits a new
-   buffer. A fast acknowledgement does **not** imply a finished render.
-4. Committed buffers change surface damage; Ferese renders the damaged output
-   and submits through EGL to the host compositor.
-
-`FERESE_TRACE_PERFORMANCE` measures Ferese's rendering/submission, not shell
-rasterization. Wayland traces distinguish software SHM buffers from GPU
-buffers (Mesa Vulkan on the tested system).
-
-## Measured validation (KDE Wayland, 175% scale)
-
-With temporary instrumentation, 24 host maximize/restore requests spaced
-250 ms apart generated 26 Winit resize events including initialization. The
-same debug binaries were tested with each renderer forced:
-
-| Measurement | tiny-skia | wgpu |
-| --- | ---: | ---: |
-| Median configure acknowledgement | 1.35 ms | 2.27 ms |
-| Median Ferese render + submit | 0.82 ms | 1.10 ms |
-| Wallpaper buffer interval | 5919 ms (only two buffers) | 9.85 ms median |
-| Bar buffers observed | 1 | 996 |
-
-The CPU capture ran for 14 seconds and the GPU capture for 18 seconds;
-counts are diagnostic, not equal-duration throughput benchmarks. GPU maximum
-buffer intervals were about 200 ms for wallpaper and 158 ms for the bar, so
-the median does not imply every frame met a refresh deadline. These figures
-include protocol tracing overhead. The temporary resize/timing hooks were
-removed after testing; no synthetic resizing runs in production.
-
-The fix does not alter output coordinates, layer exclusion, damage tracking,
-frame scheduling, or EGL swap settings. It also applies to the same shell
-running under DRM, though this validation used the nested backend.