feat: add Chapter 2 gold miner FSM example in Go

Port the finite state machine from "Programming Game AI by Example"
chapter 2: BaseGameEntity, a generic State[T]/StateMachine[T] pair
(Go generics standing in for the book's C++ templates), and a Miner
that cycles through digging, banking, sleeping, and drinking states.
VisitBankAndDepositGold, GoHomeAndSleepTilRested, and QuenchThirst
are original implementations based on the book's written behavior
description, since their C++ source wasn't available. Includes a
runnable demo in main.go.

chapter-2-state-driven-agent-design/westworld/state_quench_thirst.go

@@ -0,0 +1,53 @@
+package westworld
+
+import (
+	"fmt"
+	"sync"
+)
+
+// quenchThirst is the Go version of the (not yet pasted) singleton
+// class QuenchThirst from the book. Same pattern as
+// enterMineAndDigForNugget: unexported type, one shared instance,
+// reached only through the function below.
+type quenchThirst struct{}
+
+var (
+	quenchThirstInstance *quenchThirst
+	quenchThirstOnce     sync.Once
+)
+
+// QuenchThirst is the Go stand-in for "QuenchThirst::Instance()".
+func QuenchThirst() *quenchThirst {
+	quenchThirstOnce.Do(func() {
+		quenchThirstInstance = &quenchThirst{}
+	})
+	return quenchThirstInstance
+}
+
+var _ State[Miner] = (*quenchThirst)(nil)
+
+// The book hasn't shown us this state's real code, so this is our
+// own implementation based purely on its written description: "he
+// visits the saloon in order to buy a whiskey. When his thirst is
+// quenched, he changes state to EnterMineAndDigForNugget."
+
+// Enter sends the miner to the saloon, if he isn't there already.
+func (s *quenchThirst) Enter(m *Miner) {
+	if m.Location() != saloon {
+		fmt.Printf("\n%s: Headin' to the saloon for a whiskey", GetNameOfEntity(m.ID()))
+		m.ChangeLocation(saloon)
+	}
+}
+
+// Execute buys and drinks one whiskey, which is all it takes to
+// quench the thirst, then goes straight back to digging.
+func (s *quenchThirst) Execute(m *Miner) {
+	fmt.Printf("\n%s: That's mighty fine sippin' liquor", GetNameOfEntity(m.ID()))
+	m.BuyAndDrinkAWhiskey()
+	m.ChangeState(EnterMineAndDigForNugget())
+}
+
+// Exit announces that the miner is leaving the saloon.
+func (s *quenchThirst) Exit(m *Miner) {
+	fmt.Printf("\n%s: Leavin' the saloon, feelin' good", GetNameOfEntity(m.ID()))
+}