Show a bounded native-cache preview before authoritative reconstruction (#463)

Refs #303. file.save / file.save_as can store a warm native geometry cache (cache: true, off by default) keyed to the document hash, app version and adapter revision. Opening shows a bounded preview from it until the rebuilt session replaces it. Any cache failure counts as a miss.

crates/kernel/src/body.rs

@@ -40,7 +40,7 @@ pub struct Body {
     pub(crate) history: Option<std::sync::Arc<crate::history::History>>,
 }
 
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug, PartialEq, serde::Serialize, serde::Deserialize)]
 pub enum FaceSource {
     ExtrudeSide {
         segment: solidcraft_geom::Seg2,
@@ -110,7 +110,7 @@ impl FaceSource {
 }
 
 /// How a body looks beyond its colour, for export: its opacity and its faces' own colours.
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug, PartialEq, serde::Serialize, serde::Deserialize)]
 pub struct Paint {
     /// 1 = opaque.
     pub opacity: f32,
@@ -118,7 +118,7 @@ pub struct Paint {
 }
 
 /// A face's own colour: the B-rep face index (`face_iter` order), sRGB as 0..1, opacity.
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug, PartialEq, serde::Serialize, serde::Deserialize)]
 pub struct FacePaint {
     pub face: usize,
     pub color: [f32; 3],