test(engine): add advance_elapsed saturation and theme colour pure-function tests

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

solitaire_engine/src/game_plugin.rs

@@ -386,6 +386,14 @@ mod tests {
         assert_eq!(acc, 0.0);
     }
 
+    #[test]
+    fn advance_elapsed_saturates_at_u64_max() {
+        let mut elapsed = u64::MAX;
+        let mut acc = 0.0;
+        advance_elapsed(&mut elapsed, &mut acc, 5.0, false);
+        assert_eq!(elapsed, u64::MAX, "elapsed must not overflow past u64::MAX");
+    }
+
     #[test]
     fn advance_elapsed_handles_subsecond_deltas_without_skipping() {
         let mut elapsed = 0;

solitaire_engine/src/table_plugin.rs

@@ -244,4 +244,51 @@ mod tests {
         types.dedup();
         assert_eq!(types.len(), 13);
     }
+
+    // -----------------------------------------------------------------------
+    // Pure-function tests (no Bevy app required)
+    // -----------------------------------------------------------------------
+
+    #[test]
+    fn all_three_themes_produce_distinct_colours() {
+        let green = theme_colour(&Theme::Green);
+        let blue  = theme_colour(&Theme::Blue);
+        let dark  = theme_colour(&Theme::Dark);
+        assert_ne!(green, blue, "Green and Blue must differ");
+        assert_ne!(green, dark, "Green and Dark must differ");
+        assert_ne!(blue,  dark, "Blue and Dark must differ");
+    }
+
+    #[test]
+    fn effective_background_index_0_matches_theme_colour() {
+        for theme in [Theme::Green, Theme::Blue, Theme::Dark] {
+            let expected = theme_colour(&theme);
+            let actual   = effective_background_colour(&theme, 0);
+            assert_eq!(
+                expected, actual,
+                "index 0 must always return the theme colour for {:?}",
+                theme
+            );
+        }
+    }
+
+    #[test]
+    fn effective_background_indices_1_through_3_are_distinct_from_theme() {
+        // Non-zero indices override the theme with a fixed colour.
+        let theme_green = theme_colour(&Theme::Green);
+        for idx in 1..=3 {
+            let eff = effective_background_colour(&Theme::Green, idx);
+            assert_ne!(eff, theme_green, "index {idx} must override the theme colour");
+        }
+    }
+
+    #[test]
+    fn effective_background_index_4_falls_through_to_charcoal() {
+        // All indices ≥ 4 share the same charcoal fallback.
+        let c4 = effective_background_colour(&Theme::Green, 4);
+        let c5 = effective_background_colour(&Theme::Green, 5);
+        let c99 = effective_background_colour(&Theme::Green, 99);
+        assert_eq!(c4, c5, "indices 4 and 5 must share the charcoal fallback");
+        assert_eq!(c4, c99, "index 99 must share the charcoal fallback");
+    }
 }