Coop: stack same-type gifts on the same day into a single vote post

Multiple baskets (or multiple mimics) sent during the same day now share
one game-room post, one vote, one resolution. First-in becomes the stack
"lead"; subsequent same-type sends become followers that inherit the
lead's deadline and votes.

Behavior:
- Send a basket → new lead, new post, 6h timer starts
- Send another basket within the window → silently joins the stack,
  edits lead's post to bump count
- At stack size 2, TwinBee adds a "this gift looks REALLY special" line
  (one of 5 variants picked deterministically by lead id). No further
  escalation as the stack grows
- One !coop giftvote on the lead's id covers the whole stack
- Resolution applies the same outcome to every gift in the stack;
  modifier is N × ±6
- End-of-run gift log groups by stack with all senders attributed

Schema: coop_dungeon_gifts.stack_lead_id INTEGER (NULL for lead/standalone,
otherwise points at lead's id). Migration entry included.

Why first-in deadline (vs extending on each follower): exploitability.
Saboteurs could spam-extend a stack to delay resolution. Locked deadline
keeps senders honest about timing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -627,7 +627,9 @@ Multi-day party runs separate from solo. 2–4 players, 2–7 days depending on
 
 **Spectator betting.** Parimutuel pool, 10% rake. Bets can be placed or increased any time during the run; positions are locked in (success or failure can't be switched). Party members can bet on their own run — sandbagging while heavily bet against is exploitative and visible. Odds line shows estimated success% based on party levels, pets, neutral funding assumption, and a hidden TwinBee Helpfulness Rating (rolling last 30 floor events: how often his recommendation was right). Helpfulness shifts the line ±20% but is never shown directly — players notice the line moves after events resolve.
 
-**Gifts.** Anyone *not* in the run can spend one harvest action to send a Care Basket or a Mimic. The party never sees which type. Vote `open` or `leave`. **Each gift has its own 6-hour voting window** — voting closes when the window elapses, the gift is tallied, and the sender gets a DM with the result. The modifier sits on the run until the next floor resolution merges it in. Gifts therefore "fire" throughout the day rather than all at once at the daily tick. Magnitudes are equalized across all four outcomes — both "always open" and "always leave" yield EV=0 with σ=6, so neither is risk-averse-dominant. The choice is purely about reading sender intent.
+**Gifts.** Anyone *not* in the run can spend one harvest action to send a Care Basket or a Mimic. The party never sees which type. Vote `open` or `leave`. **Each gift's voting window is 6 hours**; voting closes when it elapses, the stack is tallied, and every sender gets a DM with the result. The modifier sits on the run until the next floor resolution merges it in. Gifts "fire" throughout the day rather than piling up at the daily tick. Magnitudes are equalized across all four outcomes — both "always open" and "always leave" yield EV=0 with σ=6, so neither is risk-averse-dominant. The choice is purely about reading sender intent.
+
+**Stacking.** Multiple gifts of the same type sent on the same day **stack into a single game-room post** — one vote covers the whole stack, modifier scales with size (5 baskets opened = +30%, 5 baskets exploded = −30%). The first-in sets the deadline; subsequent additions inherit it without extending the timer. When a stack hits size 2, TwinBee notes "this gift looks REALLY special" once and then quietly bumps the count on further additions. The end-of-run gift log groups by stack with all senders attributed.
 
 |             | Open | Leave |
 |-------------|------|-------|