Use MaterialColorSnapshot in appearance flow

Introduce unified material/color spec and tests, and refactor appearance plumbing to use MaterialColorSnapshot as the single source of truth. Add .trae material-color-service spec/checklist/tasks and integration/unit tests for plugin mapping and appearance VM behavior. AppearanceChangedEvent extended with new appearance change flags and HasChanged logic. ComponentEditorMaterialThemeAdapter rewritten to accept MaterialColorSnapshot and derive palette from snapshot data. Simplify AppearanceSettingsPageViewModel and related view code: remove legacy preview/custom-seed UI logic, preserve material/color fields when updating theme or corner radius, and update save calls to use with-expressions. Update ComponentEditorWindow to use adapter-provided OnPrimary brush and minor docs updates.

docs/VISUAL_SPEC.md

@@ -46,3 +46,11 @@ This specification defines the visual language of LanMountainDesktop, including
 - use semantic resource keys instead of hard-coded colors
 - keep glass layers visually distinct
 - maintain contrast targets for readability
+
+### Material And Color Source
+
+`IMaterialColorService` is the host source of truth for Monet seeds, wallpaper-derived colors, semantic color roles, material surfaces, and plugin appearance snapshots.
+
+New UI, component, window, and plugin appearance consumers should use `MaterialColorSnapshot` or resources produced from it. Do not recalculate application colors from raw wallpaper settings, theme settings, or `MonetPalette` in parallel.
+
+The Wallpaper settings page owns wallpaper asset/source selection. The Material & Color settings page owns color-source selection, wallpaper color-source selection, system material mode, wallpaper color refresh behavior, and color/material previews.