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_threat_test.go

@@ -150,6 +150,7 @@ func TestDeliverBriefing_AppliesThreatDrift(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	rewindToLegacyAnchor(t, exp)
 	p := &AdventurePlugin{}
 	if err := p.deliverBriefing(exp, time.Now().UTC()); err != nil {
 		t.Fatal(err)