feat(core): Snapshot derives Deserialize + PartialEq for disk persistence

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/spacesh-core/Cargo.toml

@@ -7,3 +7,6 @@ version.workspace = true
 alacritty_terminal.workspace = true
 serde.workspace = true
 spacesh-proto = { path = "../spacesh-proto" }
+
+[dev-dependencies]
+serde_json.workspace = true

crates/spacesh-core/src/snapshot.rs

@@ -1,11 +1,11 @@
-use serde::Serialize;
+use serde::{Deserialize, Serialize};
 use alacritty_terminal::index::Point;
 use alacritty_terminal::term::cell::Flags;
 use alacritty_terminal::vte::ansi::Color;
 use crate::grid::GridSurface;
 
 /// Serializable snapshot returned by `attach`.
-#[derive(Debug, Clone, Serialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct Snapshot {
     /// ANSI byte dump suitable for `xterm.write()`.
     pub ansi: String,
@@ -120,6 +120,18 @@ mod tests {
         assert_eq!(a.rows, 3);
     }
 
+    #[test]
+    fn snapshot_round_trips_through_json() {
+        let mut g = GridSurface::new(20, 4);
+        g.feed(b"hello");
+        let snap = snapshot_ansi(&g);
+        let json = serde_json::to_string(&snap).unwrap();
+        let back: Snapshot = serde_json::from_str(&json).unwrap();
+        assert_eq!(back.ansi, snap.ansi);
+        assert_eq!((back.cols, back.rows), (snap.cols, snap.rows));
+        assert_eq!((back.cursor_row, back.cursor_col), (snap.cursor_row, snap.cursor_col));
+    }
+
     #[test]
     fn cursor_is_one_based_after_input() {
         let mut g = GridSurface::new(10, 3);