N5/D2: NPC arcs — Misty stages, Robbie's gift, Thom's final letter

Three flavor arcs on existing per-player counters (no schema, golden
byte-identical):

- Misty: 3 deepening dialogue beats at MistyEncounterCount 5/15/30,
  prepended to the encounter opening the one time the counter lands on a
  threshold. Fiction only — no copy ties a donation to the hidden arena
  effects.
- Robbie: every 10th visit he leaves a consumable "for the trouble,"
  drawn from the dungeon pool at a level-matched tier.
- Thom: paying off the Tier-4 mortgage (no next tier) sends a final
  letter instead of the stock payoff line, plus an inert pet-treat
  keepsake if the player keeps a pet.

Claude-Session: https://claude.ai/code/session_017mEwUmmS7aQTP2NQXj6rUa

internal/plugin/adventure_npc_arcs_test.go

@@ -0,0 +1,91 @@
+package plugin
+
+import (
+	"strings"
+	"testing"
+)
+
+// D2 NPC arcs — pure-helper coverage. None of these touch combat, so the
+// characterization golden is unaffected; these just pin the arc thresholds,
+// gift tiers, and the keepsake shape.
+
+func TestMistyArcBeat_FiresOnlyAtThresholds(t *testing.T) {
+	want := map[int]bool{5: true, 15: true, 30: true}
+	for count := 0; count <= 40; count++ {
+		got := mistyArcBeat(count)
+		if want[count] {
+			if got == "" {
+				t.Errorf("encounter %d: expected an arc beat, got none", count)
+			}
+		} else if got != "" {
+			t.Errorf("encounter %d: expected no arc beat, got %q", count, got)
+		}
+	}
+}
+
+func TestMistyArcBeats_DoNotLeakBuffMechanics(t *testing.T) {
+	// The arena effects are a hidden discovery mechanic; arc copy must never
+	// tie a donation to a combat outcome.
+	banned := []string{"arena", "damage", "buff", "combat", "attack", "heal", "bonus"}
+	for count, beat := range mistyArcBeats {
+		lower := strings.ToLower(beat)
+		for _, b := range banned {
+			if strings.Contains(lower, b) {
+				t.Errorf("beat %d leaks mechanic word %q: %s", count, b, beat)
+			}
+		}
+	}
+}
+
+func TestRobbieGiftTier_MatchesArenaBands(t *testing.T) {
+	cases := []struct {
+		level int
+		tier  int
+	}{
+		{1, 1}, {3, 1}, {4, 2}, {7, 2}, {8, 3}, {12, 3},
+		{13, 4}, {17, 4}, {18, 5}, {20, 5},
+	}
+	for _, c := range cases {
+		if got := robbieGiftTier(c.level); got != c.tier {
+			t.Errorf("robbieGiftTier(%d) = %d, want %d", c.level, got, c.tier)
+		}
+	}
+}
+
+func TestRobbieGiftCadence(t *testing.T) {
+	// The gift lands on every 10th visit and no other.
+	for visit := 1; visit <= 35; visit++ {
+		gives := visit%robbieGiftEveryNVisits == 0
+		if gives != (visit == 10 || visit == 20 || visit == 30) {
+			t.Errorf("visit %d: unexpected gift cadence", visit)
+		}
+	}
+}
+
+func TestThomPetTreat_IsRobbieSafeKeepsake(t *testing.T) {
+	treat := thomPetTreatItem()
+	if treat.Name == "" {
+		t.Fatal("treat has no name")
+	}
+	// Robbie's sweep skips "card" items, so the keepsake survives him.
+	if treat.Type != "card" {
+		t.Errorf("treat type = %q, want card so Robbie's sweep skips it", treat.Type)
+	}
+	if treat.Value != 0 {
+		t.Errorf("treat value = %d, want 0 (inert keepsake)", treat.Value)
+	}
+	// Confirm the sweep filter actually excludes it.
+	if got := robbieQualifyingItems([]AdvItem{treat}, nil); len(got) != 0 {
+		t.Errorf("Robbie would take the treat: %+v", got)
+	}
+}
+
+func TestThomFinalLetterPet_SubstitutesPetName(t *testing.T) {
+	letter := strings.ReplaceAll(thomFinalLetterPet, "{pet}", "Biscuit")
+	if strings.Contains(letter, "{pet}") {
+		t.Error("unsubstituted {pet} placeholder remains")
+	}
+	if !strings.Contains(letter, "Biscuit") {
+		t.Error("pet name not substituted into the letter")
+	}
+}