mischief: you can now pay to have someone's expedition ruined

Mischief Makers M1 — the core engine, Matrix-only. `!mischief send elite @user`
debits the buyer, tells the games room a hit is out, and an hour later a monster
from the target's own level bracket walks into whatever dungeon they're in.
Survive it and they keep a cut of the money and the buyer is named; don't, and
they wake up on a cart home.

The monster comes from the target's bracket zone pool, not the arena ladder and
not the dungeon they happen to be standing in — the same selection code the M0
pricing sweep ran through, so the fee table can't drift away from the fight it
priced.

Three things that are load-bearing and don't look it:

  * Survival is read off the target's HP, not PlayerWon. The engine's timeout is
    a retreat, not a lethal blow — somebody who ran out the clock with HP left
    held the thing off, and a bought monster that merely outlasted them hasn't
    earned a maiming.

  * Nobody dies for money, and that includes the party. The delivery skips
    closeOutZoneWin/Loss (the fight is extrinsic to the dungeon — crediting it
    would let a buyer unlock the target's kill-gated resources for them), so
    nothing else floors a downed seat. Without floorMischiefRoster on BOTH
    outcomes, a member the leader outlived is left alive at 0 HP, which every
    `HPCurrent <= 0` gate reads as broken rather than dead.

  * One live contract per target is a partial UNIQUE INDEX, not a read-then-write
    check. Placement holds only the *buyer's* lock, so two buyers racing at the
    same victim would both pass an in-code test. The loser is refunded.

Payouts are a percentage of the base fee, never of what the buyer actually paid —
the sign surcharge is a pure sink. Capped at 75%, so a survival purse is always
strictly less than the outlay and collusion loses to !baltransfer, which is free.
That cap is the entire anti-collusion story; no danger multiplier needed.

A crash between claiming a contract and closing it out used to be unrecoverable
in the design: the row would strand, the target could never be targeted again,
and the buyer's money was gone. The stale sweep refunds those in full — that one
is our fault, not a bet they lost.

Contract timestamps bind as Go time.Time, never CURRENT_TIMESTAMP. The driver
stores RFC3339 and SQLite's own stamp is space-separated; the two compare
lexicographically wrong.

Pete learns four mischief_* event types in a separate commit, and has to deploy
BEFORE this does — an unknown event_type is a 400, which retries and then parks
the bulletin forever.

internal/plugin/adventure_worldboss_test.go

@@ -291,13 +291,13 @@ func TestWorldBossFloorHP(t *testing.T) {
 	newWorldBossTestDB(t)
 	uid := id.UserID("@floor:test.invalid")
 	fightableChar(t, uid)
-	if worldBossFloorHP(uid) {
+	if floorHPAtOne(uid) {
 		t.Error("floor should be a no-op above 0 HP")
 	}
 	if _, err := db.Get().Exec(`UPDATE dnd_character SET hp_current = 0 WHERE user_id = ?`, string(uid)); err != nil {
 		t.Fatal(err)
 	}
-	if !worldBossFloorHP(uid) {
+	if !floorHPAtOne(uid) {
 		t.Error("floor should raise a 0-HP fighter")
 	}
 	if cur, _ := dndHPSnapshot(uid); cur != 1 {