N5/D2: NPC arcs — Misty stages, Robbie's gift, Thom's final letter

Three flavor arcs on existing per-player counters (no schema, golden
byte-identical):

- Misty: 3 deepening dialogue beats at MistyEncounterCount 5/15/30,
  prepended to the encounter opening the one time the counter lands on a
  threshold. Fiction only — no copy ties a donation to the hidden arena
  effects.
- Robbie: every 10th visit he leaves a consumable "for the trouble,"
  drawn from the dungeon pool at a level-matched tier.
- Thom: paying off the Tier-4 mortgage (no next tier) sends a final
  letter instead of the stock payoff line, plus an inert pet-treat
  keepsake if the player keeps a pet.

Claude-Session: https://claude.ai/code/session_017mEwUmmS7aQTP2NQXj6rUa

internal/plugin/adventure_flavor_robbie.go

@@ -40,6 +40,11 @@ var robbieMasterworkAlreadyHas = "Oh. I see you already have one of these. Excel
 var robbieAllShopGear = "Nothing fancy today but that's alright. Clean inventory is its own reward. " +
 	"Well. The €%d is the reward. The clean inventory is a bonus. Cheerio!"
 
+// robbieLeftConsumable is appended when Robbie leaves a little something on his
+// every-10th-visit (D2). Takes the item name.
+var robbieLeftConsumable = "Oh -- one more thing. I tucked a %s into your bag on the way out. " +
+	"You've had me round enough times now that it felt rude not to. For the trouble, eh? _winks_"
+
 // ── Room Announcements ───────────────────────────────────────────────────────
 
 var robbieRoomStandard = "🎩 Robbie paid %s a visit and collected %d item(s) from their inventory. " +