feat(document): add Effect (drop shadow) data model + persistence

First slice of the Effects gap — the property panel's 效果 section
was a header-only stub with no data behind it.

- `Effect` / `DropShadow` types in `document.rs`; `Node.effects:
  Vec<Effect>` (offset_x/y + blur + color, doc-px). Both `Node`
  builders + `deep_clone_with_new_ids` carry it.
- `persistence_effects.rs` — `ShadowPayload` + `effects_to_payload`
  / `effects_from_payload`; `NodePayload.effects` is `#[serde(default)]`
  so legacy `.op` files still load. Carved into its own module so
  `persistence.rs` stays under the 800-line cap.
- Round-trip tested through the real serde JSON path (2 tests).

858 workspace tests pass. Still pending (own slices): canvas
drop-shadow paint (needs a blur primitive on `RenderBackend`),
the property-panel 效果 editing rows, and a `set_node_shadow`
MCP tool. Shadows currently round-trip through save/load but do
not yet render.

crates/openpencil-desktop/src/main.rs

@@ -16,6 +16,7 @@ mod mcp_serve;
 mod pen_doc_adapter;
 mod pen_doc_path_bounds;
 mod persistence;
+mod persistence_effects;
 mod persistence_variables;
 mod settings_io;
 

crates/openpencil-desktop/src/pen_doc_adapter.rs

@@ -621,6 +621,7 @@ fn base_payload(base: &PenNodeBase, kind: &str) -> NodePayload {
         font_size: 0.0,
         font_weight: 0,
         text_wrap: false,
+        effects: Vec::new(),
         children: Vec::new(),
     }
 }

crates/openpencil-desktop/src/persistence.rs

@@ -83,6 +83,10 @@ pub struct NodePayload {
     pub font_weight: u16,
     #[serde(default)]
     pub text_wrap: bool,
+    /// Drop-shadow effects. `#[serde(default)]` so a legacy `.op`
+    /// saved before effects round-tripped still loads (empty vec).
+    #[serde(default)]
+    pub effects: Vec<crate::persistence_effects::ShadowPayload>,
     #[serde(default)]
     pub children: Vec<NodePayload>,
 }
@@ -137,6 +141,7 @@ fn node_to_payload(n: &Node) -> NodePayload {
         font_size: n.font_size,
         font_weight: n.font_weight,
         text_wrap: n.text_wrap,
+        effects: crate::persistence_effects::effects_to_payload(&n.effects),
         children: n.children.iter().map(node_to_payload).collect(),
     }
 }
@@ -306,6 +311,7 @@ fn payload_to_node(n: NodePayload) -> Node {
     node.font_size = n.font_size;
     node.font_weight = n.font_weight;
     node.text_wrap = n.text_wrap;
+    node.effects = crate::persistence_effects::effects_from_payload(n.effects);
     node
 }
 

crates/openpencil-desktop/src/persistence_effects.rs

@@ -0,0 +1,89 @@
+//! Node-effects (drop shadow) round-trip for the desktop's private
+//! `DocPayload` save format. Carved off `persistence.rs` to keep
+//! that file under the 800-line cap.
+
+use openpencil_shell_core::document::{DropShadow, Effect};
+use serde::{Deserialize, Serialize};
+
+/// Serializable mirror of `document::DropShadow`.
+#[derive(Debug, Serialize, Deserialize)]
+pub struct ShadowPayload {
+    pub offset_x: f32,
+    pub offset_y: f32,
+    pub blur: f32,
+    pub color: [f32; 4],
+}
+
+/// Serialize a node's `effects` into payload form.
+pub fn effects_to_payload(effects: &[Effect]) -> Vec<ShadowPayload> {
+    effects
+        .iter()
+        .map(|e| {
+            let Effect::DropShadow(s) = e;
+            ShadowPayload {
+                offset_x: s.offset_x,
+                offset_y: s.offset_y,
+                blur: s.blur,
+                color: [s.color.r, s.color.g, s.color.b, s.color.a],
+            }
+        })
+        .collect()
+}
+
+/// Rebuild a node's `effects` from payload form.
+pub fn effects_from_payload(payload: Vec<ShadowPayload>) -> Vec<Effect> {
+    payload
+        .into_iter()
+        .map(|s| {
+            Effect::DropShadow(DropShadow {
+                offset_x: s.offset_x,
+                offset_y: s.offset_y,
+                blur: s.blur,
+                color: openpencil_shell_core::Color {
+                    r: s.color[0],
+                    g: s.color[1],
+                    b: s.color[2],
+                    a: s.color[3],
+                },
+            })
+        })
+        .collect()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use openpencil_shell_core::Color;
+
+    #[test]
+    fn effects_survive_payload_round_trip() {
+        let original = vec![
+            Effect::DropShadow(DropShadow {
+                offset_x: 4.0,
+                offset_y: 6.0,
+                blur: 12.0,
+                color: Color { r: 0.0, g: 0.0, b: 0.0, a: 0.5 },
+            }),
+            Effect::DropShadow(DropShadow {
+                offset_x: -2.0,
+                offset_y: 0.0,
+                blur: 3.0,
+                color: Color { r: 1.0, g: 0.0, b: 0.0, a: 1.0 },
+            }),
+        ];
+        // Through the actual serde JSON path the `.op` file uses.
+        let payload = effects_to_payload(&original);
+        let json = serde_json::to_string(&payload).expect("serialize");
+        let back: Vec<ShadowPayload> =
+            serde_json::from_str(&json).expect("deserialize");
+        let restored = effects_from_payload(back);
+        assert_eq!(restored, original);
+    }
+
+    #[test]
+    fn empty_effects_round_trip_to_empty() {
+        let payload = effects_to_payload(&[]);
+        assert!(payload.is_empty());
+        assert!(effects_from_payload(payload).is_empty());
+    }
+}

crates/openpencil-shell-core/src/document.rs

@@ -85,6 +85,26 @@ pub struct Stroke {
     pub width: f32,
 }
 
+/// Drop-shadow effect — offset + blur + colour, doc-px units.
+/// Painted behind the node's fill. Mirrors the TS `PenEffect`
+/// shadow variant (`offsetX` / `offsetY` / `blur` / `color`);
+/// `spread` is omitted until the renderer honors it.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct DropShadow {
+    pub offset_x: f32,
+    pub offset_y: f32,
+    pub blur: f32,
+    pub color: crate::Color,
+}
+
+/// A visual effect on a node. v1 ships drop shadow (the common
+/// case + what the property panel's 效果 section needs); layer
+/// blur lands once its render path does.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum Effect {
+    DropShadow(DropShadow),
+}
+
 /// Document tree node. `bounds = Rect::ZERO` for containers that
 /// derive size from children. `text` is populated for Text kind.
 #[derive(Debug, Clone)]
@@ -109,6 +129,9 @@ pub struct Node {
     pub font_size: f32,
     pub font_weight: u16,
     pub text_wrap: bool,
+    /// Visual effects (drop shadows) painted with the node. Empty
+    /// for most nodes; the property panel's 效果 section edits it.
+    pub effects: Vec<Effect>,
     pub children: Vec<Node>,
 }
 
@@ -132,6 +155,7 @@ impl Node {
             font_size: 0.0,
             font_weight: 0,
             text_wrap: false,
+            effects: Vec::new(),
             children: Vec::new(),
         }
     }
@@ -160,6 +184,7 @@ impl Node {
             font_size: 0.0,
             font_weight: 0,
             text_wrap: false,
+            effects: Vec::new(),
             children,
         }
     }

crates/openpencil-shell-core/src/document/tests_mutators.rs

@@ -538,6 +538,7 @@ fn hit_test_rotated_negative_size_line_uses_segment_midpoint_pivot() {
         font_size: 0.0,
         font_weight: 0,
         text_wrap: false,
+        effects: Vec::new(),
         children: Vec::new(),
     }];
     // Click at (50, 50) — midpoint (invariant under rotation).

crates/openpencil-shell-core/src/document/walkers.rs

@@ -549,6 +549,7 @@ pub(in crate::document) fn deep_clone_with_new_ids(node: &Node, next_id: &mut u6
         font_size: node.font_size,
         font_weight: node.font_weight,
         text_wrap: node.text_wrap,
+        effects: node.effects.clone(),
         children,
     }
 }