Expedition autopilot Phase 3: ambient between-day ticker

Active expeditions now tick on a 3-hour cadence between the 06:00
briefing and 21:00 recap. Each fire DMs the player a short TwinBee
moment — mostly pure flavor (dripping sounds, polite possums,
diplomatic moths) with a 50/50 chance of a small mechanical nudge
(±SU, +threat, 1d4 coins from a found purse, +1 HP if camped).

The dungeon is now louder while you're offline without being
balance-relevant. Multi-day expeditions feel populated by something
other than the player's calendar.

Mechanics:
- new last_ambient_at column with CAS idempotency (mirrors
  briefing/recap pattern in dnd_expedition_cycle.go)
- ambientCooldown=3h, skipped if in turn-based combat session
- 60min politeness window around scheduled briefing/recap so we
  don't pile DMs on the daily messages
- weighted event pool with per-event eligibility (camp_visitor
  requires camped, pack_rat requires SU>=1, faction_whisper
  requires threat>=30 and not siege)
- effects: applyThreatDelta, updateSupplies, euro.Credit +
  coins_earned bump, SaveDnDCharacter HP nudge (clipped at max)

Files:
- internal/db/db.go: ALTER TABLE + CREATE TABLE update
- internal/flavor/twinbee_ambient_flavor.go: 7 humor-heavy pools
- internal/plugin/expedition_ambient.go: ticker + resolver
- internal/plugin/expedition_ambient_test.go: 8 pure-logic tests
- internal/plugin/adventure.go: wire goroutine in Start()

internal/plugin/expedition_ambient_test.go

@@ -0,0 +1,150 @@
+package plugin
+
+import (
+	"math/rand/v2"
+	"strings"
+	"testing"
+	"time"
+)
+
+// Pure-logic tests for Phase 12 E7 (ambient ticker). The full
+// deliverAmbient path mutates the prod DB; those gates already live
+// behind setupAuditTestDB in other expedition tests. The behavior tested
+// here — eligibility, weighted pick, quiet-window math, DM render —
+// covers the resolution layer in isolation.
+
+func TestInAmbientQuietWindow(t *testing.T) {
+	day := func(h, m int) time.Time {
+		return time.Date(2026, 5, 14, h, m, 0, 0, time.UTC)
+	}
+	cases := []struct {
+		name string
+		t    time.Time
+		want bool
+	}{
+		{"on briefing", day(6, 0), true},
+		{"30 min before briefing", day(5, 30), true},
+		{"59 min before briefing", day(5, 1), true},
+		{"61 min before briefing", day(4, 59), false},
+		{"mid-afternoon", day(14, 17), false},
+		{"on recap", day(21, 0), true},
+		{"30 min after recap", day(21, 30), true},
+		{"61 min after recap", day(22, 1), false},
+		{"deep night", day(3, 0), false},
+	}
+	for _, c := range cases {
+		if got := inAmbientQuietWindow(c.t); got != c.want {
+			t.Errorf("%s: inAmbientQuietWindow(%v) = %v, want %v", c.name, c.t, got, c.want)
+		}
+	}
+}
+
+func TestPickAmbientEvent_AlwaysReturnsEligible(t *testing.T) {
+	// Brand-new expedition: no camp, no threat. Only "always" events
+	// should ever come back. We sample many times with different seeds
+	// to brute-force the weighted pick.
+	e := &Expedition{Supplies: ExpeditionSupplies{Current: 5}}
+	disallowed := map[string]bool{
+		"camp_visitor":    true,
+		"faction_whisper": true,
+	}
+	for i := 0; i < 200; i++ {
+		rng := rand.New(rand.NewPCG(uint64(i), uint64(i*7+1)))
+		ev := pickAmbientEvent(e, rng)
+		if disallowed[ev.Kind] {
+			t.Fatalf("seed %d picked ineligible event %q", i, ev.Kind)
+		}
+	}
+}
+
+func TestPickAmbientEvent_CampVisitorRequiresCamp(t *testing.T) {
+	e := &Expedition{
+		Supplies: ExpeditionSupplies{Current: 5},
+		Camp:     &CampState{Active: true},
+	}
+	// With camp active, camp_visitor should appear at least once in 500 spins.
+	sawVisitor := false
+	for i := 0; i < 500; i++ {
+		rng := rand.New(rand.NewPCG(uint64(i+1), uint64(i*13+2)))
+		if pickAmbientEvent(e, rng).Kind == "camp_visitor" {
+			sawVisitor = true
+			break
+		}
+	}
+	if !sawVisitor {
+		t.Errorf("expected camp_visitor to be reachable when camped")
+	}
+}
+
+func TestPickAmbientEvent_PackRatNeedsSupplies(t *testing.T) {
+	// Starving expedition: pack_rat should never fire — there's nothing
+	// for the rat to take, and a "−0.0 SU" DM would be silly.
+	e := &Expedition{Supplies: ExpeditionSupplies{Current: 0}}
+	for i := 0; i < 200; i++ {
+		rng := rand.New(rand.NewPCG(uint64(i+3), uint64(i*5+7)))
+		if pickAmbientEvent(e, rng).Kind == "pack_rat" {
+			t.Fatalf("seed %d: pack_rat fired with no supplies", i)
+		}
+	}
+}
+
+func TestPickAmbientEvent_WhisperNeedsThreat(t *testing.T) {
+	low := &Expedition{Supplies: ExpeditionSupplies{Current: 5}, ThreatLevel: 10}
+	for i := 0; i < 200; i++ {
+		rng := rand.New(rand.NewPCG(uint64(i+5), uint64(i*11+3)))
+		if pickAmbientEvent(low, rng).Kind == "faction_whisper" {
+			t.Fatalf("seed %d: whisper fired below threshold", i)
+		}
+	}
+	hot := &Expedition{Supplies: ExpeditionSupplies{Current: 5}, ThreatLevel: 60}
+	saw := false
+	for i := 0; i < 500; i++ {
+		rng := rand.New(rand.NewPCG(uint64(i+9), uint64(i*17+5)))
+		if pickAmbientEvent(hot, rng).Kind == "faction_whisper" {
+			saw = true
+			break
+		}
+	}
+	if !saw {
+		t.Errorf("expected faction_whisper to be reachable at threat 60")
+	}
+}
+
+func TestPickAmbientEvent_SiegeSuppressesWhisper(t *testing.T) {
+	// In siege mode the faction has stopped whispering and started
+	// kicking the door. The ambient whisper would undersell the moment.
+	e := &Expedition{
+		Supplies:    ExpeditionSupplies{Current: 5},
+		ThreatLevel: 100,
+		SiegeMode:   true,
+	}
+	for i := 0; i < 200; i++ {
+		rng := rand.New(rand.NewPCG(uint64(i+13), uint64(i*19+11)))
+		if pickAmbientEvent(e, rng).Kind == "faction_whisper" {
+			t.Fatalf("seed %d: whisper fired during siege", i)
+		}
+	}
+}
+
+func TestRenderAmbientDM_NoFooterPureFlavor(t *testing.T) {
+	e := &Expedition{CurrentDay: 4}
+	ev := ambientEventMonologue()
+	body := renderAmbientDM(e, ev, "A pebble blinks.", "")
+	if !strings.Contains(body, "Day 4") {
+		t.Errorf("missing day header: %q", body)
+	}
+	if !strings.Contains(body, "A pebble blinks.") {
+		t.Errorf("missing flavor line: %q", body)
+	}
+	if strings.Contains(body, "_") && strings.Count(body, "_") >= 2 {
+		t.Errorf("unexpected italic footer in pure-flavor DM: %q", body)
+	}
+}
+
+func TestRenderAmbientDM_WithFooter(t *testing.T) {
+	e := &Expedition{CurrentDay: 7}
+	body := renderAmbientDM(e, ambientEventMonologue(), "A door slams.", "Threat +1")
+	if !strings.Contains(body, "Threat +1") {
+		t.Errorf("missing footer: %q", body)
+	}
+}