chore(workspace): satisfy clippy --all-targets in test code

Five test-only lints surfaced by --all-targets were blocking CI under
-D warnings: a useless vec! in a leaderboard sort test, a
field_reassign_with_default in tuning tests, and three
assertions_on_constants in card_plugin sanity tests. The constant
assertions are now wrapped in const blocks so they run at compile time;
the runtime-formatted values were dropped from their messages because
const-block assert messages must be string literals.

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

solitaire_engine/src/card_animation/tuning.rs

@@ -210,8 +210,10 @@ mod tests {
 
     #[test]
     fn scale_duration_applies_multiplier() {
-        let mut t = AnimationTuning::default();
-        t.duration_scale = 0.5;
+        let t = AnimationTuning {
+            duration_scale: 0.5,
+            ..AnimationTuning::default()
+        };
         assert!((t.scale_duration(1.0) - 0.5).abs() < 1e-6);
         assert!((t.scale_duration(0.25) - 0.125).abs() < 1e-6);
     }

solitaire_engine/src/card_plugin.rs

@@ -1510,26 +1510,29 @@ mod tests {
 
     #[test]
     fn tableau_fan_frac_is_in_unit_interval() {
-        assert!(
-            TABLEAU_FAN_FRAC > 0.0 && TABLEAU_FAN_FRAC < 1.0,
-            "TABLEAU_FAN_FRAC must be in (0, 1), got {TABLEAU_FAN_FRAC}"
-        );
+        const {
+            assert!(
+                TABLEAU_FAN_FRAC > 0.0 && TABLEAU_FAN_FRAC < 1.0,
+                "TABLEAU_FAN_FRAC must be in (0, 1)"
+            );
+        }
     }
 
     #[test]
     fn flip_half_secs_is_positive() {
-        assert!(
-            FLIP_HALF_SECS > 0.0,
-            "FLIP_HALF_SECS must be positive, got {FLIP_HALF_SECS}"
-        );
+        const {
+            assert!(FLIP_HALF_SECS > 0.0, "FLIP_HALF_SECS must be positive");
+        }
     }
 
     #[test]
     fn font_size_frac_is_positive_and_reasonable() {
-        assert!(
-            FONT_SIZE_FRAC > 0.0 && FONT_SIZE_FRAC <= 1.0,
-            "FONT_SIZE_FRAC should be in (0, 1], got {FONT_SIZE_FRAC}"
-        );
+        const {
+            assert!(
+                FONT_SIZE_FRAC > 0.0 && FONT_SIZE_FRAC <= 1.0,
+                "FONT_SIZE_FRAC should be in (0, 1]"
+            );
+        }
     }
 
     // -----------------------------------------------------------------------

solitaire_server/src/leaderboard.rs

@@ -194,7 +194,7 @@ mod tests {
 
     #[test]
     fn leaderboard_entries_sorted_by_score_descending() {
-        let mut entries = vec![
+        let mut entries = [
             entry("Charlie", Some(1_200)),
             entry("Alice",   Some(8_000)),
             entry("Bob",     Some(3_500)),