Long expeditions D2-b: event-anchored day rollover

Splits the legacy briefing body into nightRolloverBurn + nightRolloverDrift,
plus a processNightCamp convenience. For expeditions started after the new
eventAnchoredCutoff, the 06:00 UTC briefing stops mutating: it posts a
re-engagement DM, and only force-fires processNightCamp itself after a 28h
safety-net window. Day++/burn/threat-drift now ride along the autopilot
night-camp pitch (decideAutopilotCamp sets Night=true when ≥16h since the
last rollover) and on the first player !camp since the last rollover. The
legacy UTC-anchored flow still works via the same staged helpers, with
processOvernightCamp interleaved to preserve the rest-before-drift ordering.

Tests pin eventAnchoredCutoff to year 9999 in TestMain so the existing
legacy assertions still run; new tests cover the night decision, the
night-camp pitch advancing the day, the event-anchored skip, and the
safety-net force-rollover.

internal/plugin/dnd_expedition_cycle_test.go

@@ -14,6 +14,20 @@ import (
 // synthetic "now" and verify state transitions. Ticker scheduling is a
 // thin wrapper over those.
 
+// rewindToLegacyAnchor backdates an expedition's start_date to before
+// eventAnchoredCutoff so deliverBriefing exercises the legacy UTC-anchored
+// mutator path. Tests of the D2-b event-anchored path should NOT call this.
+func rewindToLegacyAnchor(t *testing.T, exp *Expedition) {
+	t.Helper()
+	before := eventAnchoredCutoff.Add(-24 * time.Hour)
+	if _, err := db.Get().Exec(
+		`UPDATE dnd_expedition SET start_date = ? WHERE expedition_id = ?`,
+		before, exp.ID); err != nil {
+		t.Fatal(err)
+	}
+	exp.StartDate = before
+}
+
 func TestPickMorningBriefing_DayBands(t *testing.T) {
 	cases := []struct {
 		day  int
@@ -57,6 +71,7 @@ func TestDeliverBriefing_AdvancesDayAndBurnsSupplies(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	rewindToLegacyAnchor(t, exp)
 	p := &AdventurePlugin{}
 	now := time.Now().UTC()
 	if err := p.deliverBriefing(exp, now); err != nil {
@@ -87,6 +102,76 @@ func TestDeliverBriefing_AdvancesDayAndBurnsSupplies(t *testing.T) {
 	}
 }
 
+// useEventAnchored fences the eventAnchoredCutoff to before the given
+// expedition's start_date so the D2-b path is taken. Test-scoped via t.Cleanup.
+func useEventAnchored(t *testing.T, exp *Expedition) {
+	t.Helper()
+	saved := eventAnchoredCutoff
+	eventAnchoredCutoff = exp.StartDate.Add(-time.Minute)
+	t.Cleanup(func() { eventAnchoredCutoff = saved })
+}
+
+func TestDeliverBriefing_EventAnchoredSkipsMutators(t *testing.T) {
+	setupZoneRunTestDB(t)
+	uid := id.UserID("@exp-evt-skip:example")
+	defer cleanupExpeditions(uid)
+
+	exp, err := startExpedition(uid, ZoneGoblinWarrens, "",
+		ExpeditionSupplies{Current: 10, Max: 10, DailyBurn: 1, HarshMod: 1})
+	if err != nil {
+		t.Fatal(err)
+	}
+	useEventAnchored(t, exp)
+
+	// last_briefing_at is NULL and start_date is "now-ish", so the safety
+	// net should NOT fire — sub-28h since start.
+	p := &AdventurePlugin{}
+	if err := p.deliverBriefing(exp, time.Now().UTC()); err != nil {
+		t.Fatal(err)
+	}
+	got, _ := getExpedition(exp.ID)
+	if got.CurrentDay != 1 {
+		t.Errorf("day = %d, want 1 (event-anchored briefing should not advance day)", got.CurrentDay)
+	}
+	if got.Supplies.Current != 10 {
+		t.Errorf("supplies = %v, want 10 (event-anchored briefing should not burn)", got.Supplies.Current)
+	}
+}
+
+func TestDeliverBriefing_EventAnchoredSafetyNetForces(t *testing.T) {
+	setupZoneRunTestDB(t)
+	uid := id.UserID("@exp-evt-safety:example")
+	defer cleanupExpeditions(uid)
+
+	exp, err := startExpedition(uid, ZoneGoblinWarrens, "",
+		ExpeditionSupplies{Current: 10, Max: 10, DailyBurn: 1, HarshMod: 1})
+	if err != nil {
+		t.Fatal(err)
+	}
+	useEventAnchored(t, exp)
+
+	// Backdate start_date so > nightSafetyNet has elapsed with no rollover.
+	before := time.Now().UTC().Add(-(nightSafetyNet + time.Hour))
+	if _, err := db.Get().Exec(
+		`UPDATE dnd_expedition SET start_date = ? WHERE expedition_id = ?`,
+		before, exp.ID); err != nil {
+		t.Fatal(err)
+	}
+	exp.StartDate = before
+
+	p := &AdventurePlugin{}
+	if err := p.deliverBriefing(exp, time.Now().UTC()); err != nil {
+		t.Fatal(err)
+	}
+	got, _ := getExpedition(exp.ID)
+	if got.CurrentDay != 2 {
+		t.Errorf("day = %d, want 2 (safety net should force rollover)", got.CurrentDay)
+	}
+	if got.Supplies.Current >= 10 {
+		t.Errorf("supplies = %v, want < 10 (safety net should burn)", got.Supplies.Current)
+	}
+}
+
 func TestDeliverBriefing_HarshConditionsBurnFaster(t *testing.T) {
 	setupZoneRunTestDB(t)
 	uid := id.UserID("@exp-cycle-harsh:example")
@@ -97,6 +182,7 @@ func TestDeliverBriefing_HarshConditionsBurnFaster(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	rewindToLegacyAnchor(t, exp)
 	// Force harsh conditions via threat clock above 60.
 	if err := applyThreatDelta(exp.ID, 70, "test"); err != nil {
 		t.Fatal(err)