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/holdem_tip_scenarios_test.go

@@ -5,6 +5,7 @@ import (
 	"os"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/chehsunliu/poker"
 )
@@ -30,6 +31,11 @@ func loadSolverFixture() {
 
 func TestMain(m *testing.M) {
 	loadSolverFixture()
+	// D2-b: shove eventAnchoredCutoff far into the future so existing tests
+	// keep exercising the legacy UTC-anchored briefing mutator path by
+	// default. New event-anchored tests opt in by overriding the cutoff
+	// (see useEventAnchored helpers).
+	eventAnchoredCutoff = time.Date(9999, 1, 1, 0, 0, 0, 0, time.UTC)
 	os.Exit(m.Run())
 }