fix(engine): remove latent unreachable! panic in difficulty seed cursor

The Random arm was only unreachable because seeds_for(Random) returns
None in a different function — a future catalog change would turn it
into a shipped runtime panic. Returning a system-time seed is the
correct Random behaviour either way. CLAUDE.md §2.3.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/difficulty_plugin.rs

@@ -54,7 +54,10 @@ impl DifficultyIndexResource {
             DifficultyLevel::Hard => &mut self.hard,
             DifficultyLevel::Expert => &mut self.expert,
             DifficultyLevel::Grandmaster => &mut self.grandmaster,
-            DifficultyLevel::Random => unreachable!("Random has no catalog"),
+            // Random has no catalog today, so seeds_for() already returned
+            // None above; if it ever gains one, a time seed is still the
+            // right answer for "Random" — never a reachable panic.
+            DifficultyLevel::Random => return seed_from_system_time(),
         };
         let seed = catalog[*cursor % catalog.len()];
         *cursor = cursor.wrapping_add(1);