feat(card_game): add optional serde feature for Deck, Suit, Rank, Card

Adds a `serde` Cargo feature that gates #[derive(Serialize, Deserialize)]
on the four primitive types: Deck, Suit, Rank, and Card.

- Deck / Suit / Rank: plain enum derives — serialize as variant name strings
- Card: newtype over NonZeroU8 with #[serde(transparent)], serializes as
  the raw packed byte (deck<<6 | suit<<4 | rank)

Without the feature the crate remains no_std-compatible and pulls no
additional dependencies. With it, serde 1.x (derive only, no std) is
added as an optional dep.

Downstream consumers enable with:
  card_game = { ..., features = ["serde"] }

card_game/Cargo.toml

@@ -8,8 +8,12 @@ description = "Card game library."
 authors = ["Rhys Lloyd <krakow20@gmail.com>"]
 keywords = ["card", "cards", "solitaire", "klondike"]
 
+[features]
+serde = ["dep:serde"]
+
 [dependencies]
 arrayvec = { version = "0.7.6", registry = "Quaternions", features = ["len_u8"], default-features = false }
+serde = { version = "1", optional = true, default-features = false, features = ["derive"] }
 
 [lints]
 workspace = true

card_game/src/lib.rs

@@ -28,6 +28,7 @@ pub trait Game: Clone {
 
 /// card_game supports up to 4 identifiably separate decks.
 #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum Deck {
 	Deck1 = 0b00,
 	Deck2 = 0b01,
@@ -48,6 +49,7 @@ impl Deck {
 }
 
 #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum Suit {
 	Spades = 0b00,
 	Hearts = 0b01,
@@ -77,6 +79,7 @@ impl Suit {
 }
 
 #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum Rank {
 	Ace = 1,
 	Two = 2,
@@ -146,6 +149,8 @@ impl Rank {
 /// 2 bits for suit ID
 /// 4 bits for card Value
 #[derive(Clone, Debug, Eq, Hash, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[cfg_attr(feature = "serde", serde(transparent))]
 pub struct Card(core::num::NonZeroU8);
 impl Card {
 	pub const fn new(deck: Deck, suit: Suit, rank: Rank) -> Self {