refactor(core): explicit Rank discriminants, checked arithmetic, possible_instructions

- Add Rank=1..13 explicit discriminants so `rank as u8 == rank.value()`; collapse 13-arm value() match to `self as u8`
- Add Rank::RANKS and Suit::SUITS iteration constants
- Add Rank::checked_add / checked_sub (const fn, type-safe boundary enforcement); update rules.rs to use them
- Add GameState::possible_instructions() enumerating all valid move_cards triples (foundation for hints/solver)
- Fix waste buffer card peeking through during draw-slide animation by setting Visibility::Hidden on the buffer entity in sync_cards

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

solitaire_core/src/card.rs

@@ -10,6 +10,9 @@ pub enum Suit {
 }
 
 impl Suit {
+    /// All four suits in declaration order.
+    pub const SUITS: [Self; 4] = [Self::Clubs, Self::Diamonds, Self::Hearts, Self::Spades];
+
     /// Returns `true` for red suits (Diamonds, Hearts).
     pub fn is_red(self) -> bool {
         matches!(self, Suit::Diamonds | Suit::Hearts)
@@ -24,38 +27,63 @@ impl Suit {
 /// Card rank, Ace through King.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
 pub enum Rank {
-    Ace,
-    Two,
-    Three,
-    Four,
-    Five,
-    Six,
-    Seven,
-    Eight,
-    Nine,
-    Ten,
-    Jack,
-    Queen,
-    King,
+    Ace   = 1,
+    Two   = 2,
+    Three = 3,
+    Four  = 4,
+    Five  = 5,
+    Six   = 6,
+    Seven = 7,
+    Eight = 8,
+    Nine  = 9,
+    Ten   = 10,
+    Jack  = 11,
+    Queen = 12,
+    King  = 13,
 }
 
 impl Rank {
+    /// All thirteen ranks in ascending order.
+    pub const RANKS: [Self; 13] = [
+        Self::Ace, Self::Two, Self::Three, Self::Four, Self::Five,
+        Self::Six, Self::Seven, Self::Eight, Self::Nine, Self::Ten,
+        Self::Jack, Self::Queen, Self::King,
+    ];
+
     /// Numeric value: Ace = 1, King = 13.
     pub fn value(self) -> u8 {
-        match self {
-            Rank::Ace   => 1,
-            Rank::Two   => 2,
-            Rank::Three => 3,
-            Rank::Four  => 4,
-            Rank::Five  => 5,
-            Rank::Six   => 6,
-            Rank::Seven => 7,
-            Rank::Eight => 8,
-            Rank::Nine  => 9,
-            Rank::Ten   => 10,
-            Rank::Jack  => 11,
-            Rank::Queen => 12,
-            Rank::King  => 13,
+        self as u8
+    }
+
+    const fn new(n: u8) -> Option<Self> {
+        match n {
+            1  => Some(Self::Ace),
+            2  => Some(Self::Two),
+            3  => Some(Self::Three),
+            4  => Some(Self::Four),
+            5  => Some(Self::Five),
+            6  => Some(Self::Six),
+            7  => Some(Self::Seven),
+            8  => Some(Self::Eight),
+            9  => Some(Self::Nine),
+            10 => Some(Self::Ten),
+            11 => Some(Self::Jack),
+            12 => Some(Self::Queen),
+            13 => Some(Self::King),
+            _  => None,
+        }
+    }
+
+    /// Returns the rank `n` steps above `self`, or `None` if it would exceed King.
+    pub const fn checked_add(self, n: u8) -> Option<Self> {
+        Self::new((self as u8).saturating_add(n))
+    }
+
+    /// Returns the rank `n` steps below `self`, or `None` if it would go below Ace.
+    pub const fn checked_sub(self, n: u8) -> Option<Self> {
+        match (self as u8).checked_sub(n) {
+            Some(v) => Self::new(v),
+            None => None,
         }
     }
 }
@@ -79,16 +107,43 @@ mod tests {
 
     #[test]
     fn rank_values_are_sequential() {
-        let ranks = [
-            Rank::Ace, Rank::Two, Rank::Three, Rank::Four, Rank::Five,
-            Rank::Six, Rank::Seven, Rank::Eight, Rank::Nine, Rank::Ten,
-            Rank::Jack, Rank::Queen, Rank::King,
-        ];
-        for (i, r) in ranks.iter().enumerate() {
+        for (i, r) in Rank::RANKS.iter().enumerate() {
             assert_eq!(r.value(), (i + 1) as u8);
         }
     }
 
+    #[test]
+    fn rank_as_u8_matches_value() {
+        for r in Rank::RANKS {
+            assert_eq!(r as u8, r.value());
+        }
+    }
+
+    #[test]
+    fn rank_checked_add_boundary() {
+        assert_eq!(Rank::King.checked_add(1), None);
+        assert_eq!(Rank::Queen.checked_add(1), Some(Rank::King));
+        assert_eq!(Rank::Ace.checked_add(1), Some(Rank::Two));
+        assert_eq!(Rank::Five.checked_add(3), Some(Rank::Eight));
+    }
+
+    #[test]
+    fn rank_checked_sub_boundary() {
+        assert_eq!(Rank::Ace.checked_sub(1), None);
+        assert_eq!(Rank::Two.checked_sub(1), Some(Rank::Ace));
+        assert_eq!(Rank::King.checked_sub(1), Some(Rank::Queen));
+        assert_eq!(Rank::Five.checked_sub(3), Some(Rank::Two));
+    }
+
+    #[test]
+    fn suit_suits_contains_all_four() {
+        assert_eq!(Suit::SUITS.len(), 4);
+        assert!(Suit::SUITS.contains(&Suit::Clubs));
+        assert!(Suit::SUITS.contains(&Suit::Diamonds));
+        assert!(Suit::SUITS.contains(&Suit::Hearts));
+        assert!(Suit::SUITS.contains(&Suit::Spades));
+    }
+
     #[test]
     fn suit_red_and_black_are_complementary() {
         for suit in [Suit::Clubs, Suit::Diamonds, Suit::Hearts, Suit::Spades] {