adventure: tell Pete this build knows what a party seat is

The party list is omitempty, so a solo run and a game box too old to push
seats reach Pete as the same empty slice, and those two want opposite
buttons. Pete's page was reading the empty one as "solo, therefore leader"
and offering a party member the button that throws away everyone's day.

One boolean, set on every sheet a build makes — in town, solo, or seated
with three others. It says something about the sender, never about the
character, so it is set unconditionally: a conditional flag would read as
"this player is solo" and land straight back in the hole it closes. An
older build sends no key at all, which decodes to false, which is the
answer Pete wants when it cannot tell.

Pete's half shipped already and withholds the button until this arrives.

Verified end to end against a local Pete, not just in tests: a solo leader
on a live expedition gets "Call the whole thing off" back, a member still
gets "Turn back alone" and never the abandon, and stripping the flag from
the same push takes the solo button away again.

internal/plugin/pete_roster_party_test.go

@@ -1,6 +1,8 @@
 package plugin
 
 import (
+	"encoding/json"
+	"strings"
 	"testing"
 	"time"
 
@@ -178,6 +180,71 @@ func TestOptedOutSeatIsAnonymisedNotDropped(t *testing.T) {
 	}
 }
 
+// TestPartyKnownIsSetOnEverySheet pins the capability flag Pete's abandon button
+// hangs off. Party is omitempty, so a solo run and a game box too old to know what
+// a seat is both reach Pete as an empty slice; the flag is what tells them apart.
+// It is a fact about this build, so it must be true on a sheet with no party on it
+// at all — a conditional party_known reads as "this player is solo" and puts the
+// flag back in the hole it was added to close.
+func TestPartyKnownIsSetOnEverySheet(t *testing.T) {
+	newBoredomTestDB(t)
+	now := time.Now().UTC()
+	old := now.Add(-30 * time.Hour)
+
+	intown := id.UserID("@intown:test")
+	solo := id.UserID("@solo:test")
+	leader := id.UserID("@leader:test")
+	member := id.UserID("@member:test")
+	seedRosterPlayer(t, intown, "Nonk", &old, &old)
+	seedRosterPlayer(t, solo, "Quack", &old, &old)
+	seedRosterPlayer(t, leader, "Josie", &old, &old)
+	seedRosterPlayer(t, member, "Camcast", &old, &old)
+
+	seedExpedition(t, "exp-solo", solo, "active")
+	seedExpedition(t, "exp-shared", leader, "active")
+	seatLeaderFixture(t, "exp-shared")
+	if err := joinParty("exp-shared", member); err != nil {
+		t.Fatalf("joinParty: %v", err)
+	}
+
+	snap, err := buildRosterSnapshot(now, nil)
+	if err != nil {
+		t.Fatalf("buildRosterSnapshot: %v", err)
+	}
+	var seen int
+	for _, a := range snap.Adventurers {
+		if a.Detail == nil {
+			t.Fatalf("%s has no detail sheet to carry the flag", a.Name)
+		}
+		seen++
+		if !a.Detail.PartyKnown {
+			t.Errorf("%s (%s, %d seats) published party_known=false; this build knows what a seat is",
+				a.Name, a.Status, len(a.Detail.Party))
+		}
+	}
+	if seen != 4 {
+		t.Fatalf("checked %d sheets, want 4 — a case went missing", seen)
+	}
+
+	// The wire name is the contract: Pete decodes party_known and withholds the
+	// abandon button when it is absent, so a rename here fails silently and only
+	// on the far side.
+	blob, err := json.Marshal(&peteclient.RosterDetail{PartyKnown: true})
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	if !strings.Contains(string(blob), `"party_known":true`) {
+		t.Errorf("detail sheet serialised without party_known: %s", blob)
+	}
+	// And it must not be omitempty: an absent key is Pete's fail-closed answer,
+	// which a false flag has to keep meaning.
+	if blob, err = json.Marshal(&peteclient.RosterDetail{}); err != nil {
+		t.Fatalf("marshal: %v", err)
+	} else if !strings.Contains(string(blob), `"party_known":false`) {
+		t.Errorf("party_known is omitempty; false must stay on the wire: %s", blob)
+	}
+}
+
 // seatsForOwner pulls one named adventurer's published party out of a whole
 // snapshot, which is the only way to reach it — Party rides RosterDetail, so this
 // also proves the wiring in buildRosterSnapshot and not just partySeatViews.