feat(data,sync,engine): per-mode best score and fastest win

Lifetime stats now also track best score and fastest win per game
mode (Classic, Zen, Challenge), additive on top of the existing
all-modes-combined `best_single_score` and `fastest_win_seconds`.
Time Attack is intentionally excluded — its scoring model is
session-level (count of wins inside a 10-minute window) so a
per-game best wouldn't compose. Daily Challenge inherits Classic
scoring and contributes through the Classic row.

- `solitaire_sync::StatsSnapshot` gains six fields (`{mode}_best_score`,
  `{mode}_fastest_win_seconds` × {Classic, Zen, Challenge}). All are
  `#[serde(default)]` so older save files load cleanly to zeros.
- `solitaire_sync::merge` propagates the per-mode bests through the
  same max/min logic as the global counterparts.
- `solitaire_data::StatsExt::update_per_mode_bests` is the engine's
  entry point — called from `update_stats_on_win` alongside the
  existing `update_on_win`.
- Stats overlay grows a "Per-mode bests" section with three rows
  (Classic / Zen / Challenge) tagged with `PerModeBestsRow`. Empty
  rows render an em-dash, matching the first-launch zero-state
  treatment used by the primary cells.
- 3 new tests cover the rendering, the Classic-mode update path,
  and the Zen-mode update path.

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

solitaire_sync/src/stats.rs

@@ -33,6 +33,56 @@ pub struct StatsSnapshot {
     pub draw_one_wins: u32,
     /// Wins achieved in Draw-Three mode.
     pub draw_three_wins: u32,
+
+    // -----------------------------------------------------------------
+    // Per-mode bests
+    //
+    // These mirror `best_single_score` / `fastest_win_seconds` but
+    // narrowed to one [`solitaire_core::game_state::GameMode`]. They are
+    // additive: lifetime totals continue to track across all modes, and
+    // legacy `stats.json` files load to 0 for every new field via
+    // `#[serde(default)]`.
+    //
+    // Time-Attack and Daily-Challenge are intentionally absent here:
+    // - Time Attack has its own session-level scoring (count of wins
+    //   inside a 10-minute window); a per-game best wouldn't compose.
+    // - Daily Challenge uses Classic scoring rules and so already
+    //   contributes to `classic_*` here.
+    //
+    // Sentinel for `*_fastest_win_seconds` is `0` (not `u64::MAX`),
+    // because legacy files deserialise unknown fields to the type's
+    // `Default::default()` — and `u64::default()` is 0. The merge logic
+    // and the UI must therefore treat 0 as "no win recorded yet".
+    // -----------------------------------------------------------------
+
+    /// Best single score achieved in Classic mode (Draw-One or Draw-Three).
+    /// 0 means "no Classic win yet".
+    #[serde(default)]
+    pub classic_best_score: u32,
+
+    /// Fastest Classic-mode win time, in seconds. 0 means "no Classic win yet".
+    #[serde(default)]
+    pub classic_fastest_win_seconds: u64,
+
+    /// Best single score achieved in Zen mode. Zen has no time pressure but
+    /// scoring is still on, so players who care about it still play for a high.
+    /// 0 means "no Zen win yet".
+    #[serde(default)]
+    pub zen_best_score: u32,
+
+    /// Fastest Zen-mode win time, in seconds. 0 means "no Zen win yet".
+    #[serde(default)]
+    pub zen_fastest_win_seconds: u64,
+
+    /// Best single score achieved in Challenge mode (the hardest mode — separate
+    /// leaderboard). 0 means "no Challenge win yet".
+    #[serde(default)]
+    pub challenge_best_score: u32,
+
+    /// Fastest Challenge-mode win time, in seconds. 0 means "no Challenge win yet".
+    #[serde(default)]
+    pub challenge_fastest_win_seconds: u64,
+
     /// Wall-clock time of the last modification (used for conflict detection).
     pub last_modified: DateTime<Utc>,
 }
@@ -51,6 +101,12 @@ impl Default for StatsSnapshot {
             best_single_score: 0,
             draw_one_wins: 0,
             draw_three_wins: 0,
+            classic_best_score: 0,
+            classic_fastest_win_seconds: 0,
+            zen_best_score: 0,
+            zen_fastest_win_seconds: 0,
+            challenge_best_score: 0,
+            challenge_fastest_win_seconds: 0,
             last_modified: DateTime::UNIX_EPOCH,
         }
     }
@@ -147,4 +203,20 @@ mod tests {
         assert_eq!(s.win_streak_best, 7, "best streak must not be reduced on abandon");
         assert_eq!(s.win_streak_current, 0);
     }
+
+    #[test]
+    fn per_mode_fields_default_to_zero() {
+        // The new per-mode fields must default to 0 — both in the explicit
+        // `Default` impl and (because of `#[serde(default)]`) for any
+        // legacy payload that omits them. The legacy-JSON deserialise
+        // round-trip lives in `solitaire_data::stats` where `serde_json`
+        // is in scope.
+        let s = StatsSnapshot::default();
+        assert_eq!(s.classic_best_score, 0);
+        assert_eq!(s.classic_fastest_win_seconds, 0);
+        assert_eq!(s.zen_best_score, 0);
+        assert_eq!(s.zen_fastest_win_seconds, 0);
+        assert_eq!(s.challenge_best_score, 0);
+        assert_eq!(s.challenge_fastest_win_seconds, 0);
+    }
 }