feat(klondike): add optional serde feature for instruction/pile serialization

Adds serde Serialize/Deserialize derives (behind the `serde` feature flag)
to Tableau, Foundation, KlondikePile, SkipCards, TableauStack,
KlondikePileStack, DstFoundation, DstTableau, and KlondikeInstruction.

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

klondike/Cargo.toml

@@ -3,9 +3,13 @@ name = "klondike"
 version = "0.3.0"
 edition = "2024"
 
+[features]
+serde = ["dep:serde"]
+
 [dependencies]
 card_game.workspace = true
 rand = { version = "0.10.1", default-features = false, features = ["std_rng"] }
+serde = { version = "1", optional = true, default-features = false, features = ["derive"] }
 
 [lints]
 workspace = true

klondike/src/lib.rs

@@ -121,6 +121,7 @@ impl KlondikeStats {
 }
 
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum Tableau {
 	Tableau1,
 	Tableau2,
@@ -147,6 +148,7 @@ impl Tableau {
 }
 
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum Foundation {
 	Foundation1,
 	Foundation2,
@@ -167,6 +169,7 @@ impl Foundation {
 }
 
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum KlondikePile {
 	Tableau(Tableau),
 	Stock,
@@ -200,6 +203,7 @@ impl From<Foundation> for KlondikePile {
 }
 
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum SkipCards {
 	Skip0,
 	Skip1,
@@ -238,6 +242,7 @@ impl SkipCards {
 }
 
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct TableauStack {
 	pub tableau: Tableau,
 	pub skip_cards: SkipCards,
@@ -271,6 +276,7 @@ impl TableauStack {
 }
 
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum KlondikePileStack {
 	Tableau(TableauStack),
 	Stock,
@@ -294,6 +300,7 @@ impl KlondikePileStack {
 }
 
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct DstFoundation {
 	pub src: KlondikePile,
 	pub foundation: Foundation,
@@ -317,6 +324,7 @@ impl DstFoundation {
 }
 
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct DstTableau {
 	pub src: KlondikePileStack,
 	pub tableau: Tableau,
@@ -340,6 +348,7 @@ impl DstTableau {
 }
 
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum KlondikeInstruction {
 	DstFoundation(DstFoundation),
 	DstTableau(DstTableau),