adventure: tell Pete who else is on the expedition

Party seats ride the roster detail, beside the supply and threat numbers the
same expedition already publishes. Leader first, the hireling named without a
token he has no board row for, and an opted-out player's seat kept and
anonymised — dropping it would make a party of three read as a pair while the
burn rate and enemy scaling beside it still felt three bodies.

The board itself was wrong about this: it resolved an expedition with a lookup
keyed on dnd_expedition.user_id, which is blind to members, so for the whole
life of N3 parties a seated player has shown as idle in town while standing in
a dungeon.

Pets also send the engine's XP threshold for their current level band, so the
web can draw progress without keeping a copy of a curve that would drift.

internal/plugin/pete_detail_test.go

@@ -226,3 +226,55 @@ func TestDetailSnapshotSkipsDeadPlayers(t *testing.T) {
 		t.Fatalf("detail set = %+v, want just the living player", snap.Players)
 	}
 }
+
+// TestPetXPRidesTheCurveNotACopyOfIt pins the unit and the cap, which are the two
+// ways this can go quietly wrong on the web. XP is stored in centi-XP — a pet
+// earns 1.5 points an action and the ledger is an int — so a page that read XP
+// against a whole-number threshold would draw a bar 100x too full. And a capped
+// pet must report 0 needed rather than the next band's number, or its bar sits
+// forever short of a level it can never gain.
+func TestPetXPRidesTheCurveNotACopyOfIt(t *testing.T) {
+	newMischiefTestDB(t)
+	uid := id.UserID("@quack:test")
+	seedDetailPlayer(t, uid, "Quack", 7)
+
+	adv, err := loadAdvCharacter(uid)
+	if err != nil {
+		t.Fatalf("loadAdvCharacter: %v", err)
+	}
+	adv.PetType = "cat"
+	adv.PetName = "Mittens"
+	adv.PetLevel = 4
+	adv.PetXP = 750 // 7.5 of the 20 points level 4 wants
+	adv.Pet2Type = "dog"
+	adv.Pet2Name = "Rex"
+	adv.Pet2Level = petMaxLevel
+	adv.Pet2XP = 0
+	if err := saveAdvCharacter(adv); err != nil {
+		t.Fatalf("saveAdvCharacter: %v", err)
+	}
+
+	snap, err := (&AdventurePlugin{}).buildDetailSnapshot(time.Now().UTC())
+	if err != nil {
+		t.Fatalf("buildDetailSnapshot: %v", err)
+	}
+	pets := snap.Players[0].Pets
+	if len(pets) != 2 {
+		t.Fatalf("pets = %+v, want both slots", pets)
+	}
+	byName := map[string]int{}
+	for i, p := range pets {
+		byName[p.Name] = i
+	}
+	mittens := pets[byName["Mittens"]]
+	if mittens.XP != 750 {
+		t.Errorf("Mittens XP = %d, want the stored centi-XP 750", mittens.XP)
+	}
+	if want := petXPToNextLevel(4) * 100; mittens.XPNeeded != want {
+		t.Errorf("Mittens XPNeeded = %d, want %d — the curve is in centi-XP too",
+			mittens.XPNeeded, want)
+	}
+	if rex := pets[byName["Rex"]]; rex.XPNeeded != 0 {
+		t.Errorf("a capped pet needs %d more XP; want 0, meaning nothing left to earn", rex.XPNeeded)
+	}
+}