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_audit_phase_R7_test.go

@@ -168,9 +168,7 @@ func TestDeliverBriefing_StarvationForcesExtraction(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if _, err := dbExecExpeditionBackdate(exp.ID, 24*time.Hour); err != nil {
-		t.Fatalf("backdate: %v", err)
-	}
+	rewindToLegacyAnchor(t, exp)
 
 	p := &AdventurePlugin{}
 	now := time.Date(2026, 5, 8, 6, 0, 0, 0, time.UTC)