Fix NPC audit findings: race conditions, debit checks, sniper mechanics

Concurrency:
- npcTrackMessage now acquires advUserLock to prevent lost updates
  on message count and double encounter triggers from rapid messages
- Lock released before spawning encounter goroutine (avoids deadlock)
- Only one NPC encounter fires per message

Security:
- Debit() return value checked for both Misty and Arina payments
- Buff only applied on successful debit; failed debit falls through
  to decline/debuff path
- NPC encounter won't overwrite existing pending interactions (shop,
  treasure discard, etc.)

Arena sniper rework:
- Sniper checked independently before combat roll, not after
- If sniper fires, combat roll is skipped entirely (not a death save)
- Sniper kill renders as standalone narrative with no combat log
- Sniper flavor text rewritten: reads as random arena chaos with no
  hint of Arina's involvement

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/plugin/adventure_arena.go

@@ -139,24 +139,24 @@ func (p *AdventurePlugin) handleArenaFight(ctx MessageContext) error {
 		return p.SendDM(ctx.Sender, "Arena data error. This shouldn't happen.")
 	}
 
-	// Resolve combat
+	// NPC arena effects — sniper checked first (independent of combat roll)
+	npcResult := npcCheckArenaEffects(char, monster.Name)
+
+	if npcResult != nil && npcResult.SniperKill {
+		// Sniper fired — enemy dies, skip combat roll entirely
+		combatLog := &ArenaCombatLog{PlayerHP: 100, EnemyHP: 0, PlayerWon: true}
+		return p.resolveArenaSurvival(ctx, run, char, tier, monster, combatLog, npcResult)
+	}
+
+	// Normal combat roll
 	deathChance := arenaDeathChance(monster, char, equip)
 	roll := rand.Float64()
 	died := roll < deathChance
 
-	// Generate combat log (cosmetic — outcome already determined)
 	closeness := 1.0 - math.Abs(roll-deathChance)/math.Max(deathChance, 1-deathChance)
 	combatLog := generateArenaCombatLog(!died, closeness)
 
-	// NPC arena effects (Misty/Arina) — checked after combat roll
-	npcResult := npcCheckArenaEffects(char, monster.Name)
-	if npcResult != nil && npcResult.SniperKill {
-		// Sniper overrides death — enemy is killed instantly
-		died = false
-	}
-
 	if died {
-		// Apply crowd revenge text to death message if applicable
 		if npcResult != nil && npcResult.Text != "" {
 			combatLog.NPCText = npcResult.Text
 		}
@@ -205,6 +205,13 @@ func (p *AdventurePlugin) confirmAndStartArenaRun(ctx MessageContext) error {
 	equip, _ := loadAdvEquipment(ctx.Sender)
 	monster := arenaGetMonster(1, 1)
 
+	npcResult := npcCheckArenaEffects(char, monster.Name)
+
+	if npcResult != nil && npcResult.SniperKill {
+		combatLog := &ArenaCombatLog{PlayerHP: 100, EnemyHP: 0, PlayerWon: true}
+		return p.resolveArenaSurvival(ctx, run, char, tier, monster, combatLog, npcResult)
+	}
+
 	deathChance := arenaDeathChance(monster, char, equip)
 	roll := rand.Float64()
 	died := roll < deathChance
@@ -212,11 +219,6 @@ func (p *AdventurePlugin) confirmAndStartArenaRun(ctx MessageContext) error {
 	closeness := 1.0 - math.Abs(roll-deathChance)/math.Max(deathChance, 1-deathChance)
 	combatLog := generateArenaCombatLog(!died, closeness)
 
-	npcResult := npcCheckArenaEffects(char, monster.Name)
-	if npcResult != nil && npcResult.SniperKill {
-		died = false
-	}
-
 	if died {
 		if npcResult != nil && npcResult.Text != "" {
 			combatLog.NPCText = npcResult.Text
@@ -321,18 +323,24 @@ func (p *AdventurePlugin) resolveArenaSurvival(ctx MessageContext, run *ArenaRun
 	}
 	run.XPAccumulated += battleXP
 
-	// Build survival message with combat log
+	// Build survival message
+	var text string
+	if npcResult != nil && npcResult.SniperKill {
+		// Sniper killed the enemy — no combat, just the sniper line
+		text = npcResult.Text + "\n\n"
+		text += fmt.Sprintf("🏆 +%d XP | €%d earned\n", battleXP, reward)
+	} else {
 		closer := arenaWinCloser(monster.Name, len(combatLog.Rounds))
-	text := renderArenaCombatLog(combatLog, monster, true, reward, battleXP, closer)
+		text = renderArenaCombatLog(combatLog, monster, true, reward, battleXP, closer)
 
-	// Append NPC effects (Misty food/crowd, Arina sniper)
+		// Append NPC effects (Misty food/crowd)
 		if npcResult != nil && npcResult.Text != "" {
 			text += npcResult.Text
-		// Apply equipment condition repair from gourmet food
 			if npcResult.CondRepair > 0 {
 				npcRepairMostDamaged(ctx.Sender, equip, npcResult.CondRepair)
 			}
 		}
+	}
 
 	text += fmt.Sprintf("\nTier earnings: €%d | Session total: €%d (at risk)\n",
 		run.TierEarnings, run.Earnings+run.TierEarnings)

internal/plugin/adventure_flavor_npcs.go

@@ -77,9 +77,9 @@ var mistyCrowdRevengeLines = []string{
 // ── Sniper Log Lines (Arina buff — arena) ──────────────────────────────────
 
 var arinaSniperLines = []string{
-	"Something arrives from the upper tier. {enemy} doesn't finish their round.",
-	"A bolt. One bolt. {enemy} is down. Nobody in the crowd reacts. They saw nothing.",
-	"The shot comes from somewhere in the stands. {enemy} had a bad week. It ended here.",
-	"From the upper tier: one shot. {enemy} goes down. The fight continues without them.",
-	"The arena lights caught it briefly. Just briefly. {enemy} is down.",
+	"Just as the match was about to begin, something whizzes inches from the side of your head into {enemy}. They look confused because they've been shot with an arrow, yet you're holding a sword. Oh well. The Arena can be brutal sometimes. YOU WIN!",
+	"You step forward to fight. {enemy} steps forward to meet you. Then {enemy} stumbles. There's an arrow in their shoulder that wasn't there a second ago. Neither of you saw where it came from. {enemy} goes down. You didn't do that. YOU WIN!",
+	"The bell rings. You raise your weapon. {enemy} raises theirs. Then something whistles past your ear and {enemy} drops. You stand there for a moment. The crowd cheers like this is normal. Maybe it is. YOU WIN!",
+	"Before you can swing, {enemy} jerks sideways. There's a bolt sticking out of them that absolutely did not come from you. The crowd doesn't seem surprised. {enemy} is down. You'll take it. YOU WIN!",
+	"You blink. {enemy} is on the ground. There's an arrow in their leg and a confused expression on their face. You definitely didn't do that. The Arena official shrugs and waves you through. YOU WIN!",
 }

internal/plugin/adventure_npcs.go

@@ -41,8 +41,14 @@ const (
 // It increments their daily message count and fires NPC encounter rolls
 // when thresholds are hit.
 func (p *AdventurePlugin) npcTrackMessage(userID id.UserID) {
+	// Acquire user lock to prevent lost updates on message count and
+	// double encounter triggers from rapid messages.
+	userMu := p.advUserLock(userID)
+	userMu.Lock()
+
 	char, err := loadAdvCharacter(userID)
 	if err != nil || !char.Alive {
+		userMu.Unlock()
 		return
 	}
 
@@ -58,39 +64,36 @@ func (p *AdventurePlugin) npcTrackMessage(userID id.UserID) {
 
 	char.NPCMsgCount++
 
+	var fireNPC string
+
 	// Check Misty threshold
 	if char.MistyRollTarget > 0 && char.NPCMsgCount == char.MistyRollTarget {
 		if p.npcShouldEncounter(char, "misty") {
-			// Zero the target so we don't re-trigger
 			char.MistyRollTarget = 0
-			if err := saveAdvCharacter(char); err != nil {
-				slog.Error("npc: failed to save char after misty trigger", "user", userID, "err", err)
-			}
-			safeGo("npc-misty-encounter", func() {
-				p.npcFireEncounter(userID, "misty")
-			})
-			return
+			fireNPC = "misty"
 		}
 	}
 
-	// Check Arina threshold
-	if char.ArinaRollTarget > 0 && char.NPCMsgCount == char.ArinaRollTarget {
+	// Check Arina threshold (only if Misty didn't fire — one encounter per message)
+	if fireNPC == "" && char.ArinaRollTarget > 0 && char.NPCMsgCount == char.ArinaRollTarget {
 		if p.npcShouldEncounter(char, "arina") {
 			char.ArinaRollTarget = 0
-			if err := saveAdvCharacter(char); err != nil {
-				slog.Error("npc: failed to save char after arina trigger", "user", userID, "err", err)
-			}
-			safeGo("npc-arina-encounter", func() {
-				p.npcFireEncounter(userID, "arina")
-			})
-			return
+			fireNPC = "arina"
 		}
 	}
 
-	// Save updated count
 	if err := saveAdvCharacter(char); err != nil {
 		slog.Error("npc: failed to save msg count", "user", userID, "err", err)
 	}
+
+	// Release lock BEFORE firing encounter (npcFireEncounter re-acquires it)
+	userMu.Unlock()
+
+	if fireNPC != "" {
+		safeGo("npc-"+fireNPC+"-encounter", func() {
+			p.npcFireEncounter(userID, fireNPC)
+		})
+	}
 }
 
 // npcShouldEncounter checks cooldown and rolls the 7.5% chance.
@@ -145,6 +148,11 @@ func (p *AdventurePlugin) npcFireEncounter(userID id.UserID, npc string) {
 		return
 	}
 
+	// Don't overwrite an existing pending interaction (shop, treasure, etc.)
+	if _, occupied := p.pending.Load(string(userID)); occupied {
+		return
+	}
+
 	// Set pending interaction
 	p.pending.Store(string(userID), &advPendingInteraction{
 		Type:      "npc_encounter",
@@ -206,7 +214,13 @@ func (p *AdventurePlugin) resolveMisty(ctx MessageContext, char *AdventureCharac
 			return p.SendDM(ctx.Sender, "You reach for your gold but there isn't enough. She sees this. She doesn't say anything.\n\n_"+mistyDeclineLine+"_")
 		}
 
-		p.euro.Debit(ctx.Sender, float64(mistyCost), "misty_donation")
+		if !p.euro.Debit(ctx.Sender, float64(mistyCost), "misty_donation") {
+			char.MistyDebuffExpires = expires
+			if err := saveAdvCharacter(char); err != nil {
+				slog.Error("npc: failed to save misty debuff", "user", ctx.Sender, "err", err)
+			}
+			return p.SendDM(ctx.Sender, "You reach for your gold but there isn't enough. She sees this. She doesn't say anything.\n\n_"+mistyDeclineLine+"_")
+		}
 		char.MistyBuffExpires = expires
 		if err := saveAdvCharacter(char); err != nil {
 			slog.Error("npc: failed to save misty buff", "user", ctx.Sender, "err", err)
@@ -232,7 +246,10 @@ func (p *AdventurePlugin) resolveArina(ctx MessageContext, char *AdventureCharac
 			return p.SendDM(ctx.Sender, fmt.Sprintf("You can't afford it. She noticed before you did.\n\n_%s_", reply))
 		}
 
-		p.euro.Debit(ctx.Sender, float64(arinaCost), "arina_investment")
+		if !p.euro.Debit(ctx.Sender, float64(arinaCost), "arina_investment") {
+			reply := arinaDeclineLines[rand.IntN(len(arinaDeclineLines))]
+			return p.SendDM(ctx.Sender, fmt.Sprintf("You can't afford it. She noticed before you did.\n\n_%s_", reply))
+		}
 		char.ArinaBuffExpires = expires
 		if err := saveAdvCharacter(char); err != nil {
 			slog.Error("npc: failed to save arina buff", "user", ctx.Sender, "err", err)