feat(core): flesh out SecureNoteCore (Zeroizing body)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/idfoto-core/src/item_types/secure_note.rs

@@ -1,3 +1,30 @@
+//! Secure note: just a multiline body, Zeroizing.
+
 use serde::{Deserialize, Serialize};
+use zeroize::Zeroizing;
+
 #[derive(Debug, Clone, Serialize, Deserialize, Default)]
-pub struct SecureNoteCore {}
+pub struct SecureNoteCore {
+    pub body: Zeroizing<String>,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn secure_note_round_trips() {
+        let note = SecureNoteCore { body: Zeroizing::new("a multi\nline note".into()) };
+        let json = serde_json::to_string(&note).unwrap();
+        let parsed: SecureNoteCore = serde_json::from_str(&json).unwrap();
+        assert_eq!(parsed.body.as_str(), "a multi\nline note");
+    }
+
+    #[test]
+    fn empty_body_round_trips() {
+        let note = SecureNoteCore::default();
+        let json = serde_json::to_string(&note).unwrap();
+        let parsed: SecureNoteCore = serde_json::from_str(&json).unwrap();
+        assert!(parsed.body.is_empty());
+    }
+}