refactor(core): add missing derives to AchievementContext (M-20)

Add PartialEq, Eq, Serialize, Deserialize to AchievementContext per
CLAUDE.md §5.3 derive order. The struct holds only primitive types
(u32, u64, i32, bool, Option<u32>) so all four derives apply without
complications.

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

solitaire_core/src/achievement.rs

@@ -8,9 +8,11 @@
 //! walks `ALL_ACHIEVEMENTS`, evaluates each `condition`, and emits an
 //! unlock event for any `AchievementDef` whose record is not yet unlocked.
 
+use serde::{Deserialize, Serialize};
+
 /// Fields needed by achievement conditions. Constructed by the engine from
 /// `StatsSnapshot`, the final `GameState`, and wall-clock time.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct AchievementContext {
     /// Total number of games played (after this win has been recorded).
     pub games_played: u32,