Combat engine §2(a): the enemy charges for what a seat brings

Both scaling levers counted seats. partyEnemyHPScale gave +15% boss HP for any
roster >= 2, and partyActionExpectation lifted the enemy from 1 to 2.4 attacks a
round. A seat COUNT charges the same for an under-levelled friend, a hired NPC,
and a true peer — so a below-median body cost a full seat's worth of boss and did
not give a full seat's worth back.

Measured, once the companion's free full-heal was taken away and he became honest:
hiring him was WORSE than going alone (66.1% against solo's 69.0%). That is this
bug, and it has been live for every under-levelled friend anyone has ever invited.

Seats now carry a SeatWeight, and both levers scale on the summed weight of the
LIVING seats rather than on a head count. The weight is level-based, priced against
the leader, times a discount for a hireling (no subclass, no magic items, gear that
is never Masterwork — the layers a player accrues and a hireling never will).

Level, and deliberately not a power score: an HP-x-damage proxy would rank a cleric
below a fighter and quietly make every mixed HUMAN party easier, which is a
difficulty regression smuggled in under a bug fix.

The safety argument is one property: **a peer weighs exactly 1.0**. So the curves
interpolate between the integer knots the P8 sweep tuned — (1, 1.0), (2, 2.4),
2n-1 from 3 up — and every integer input returns exactly what it always returned.
Solo is byte-identical, a party of same-level humans is byte-identical, both
goldens hold unmoved, and only an UNEQUAL roster lands between the knots. That is
the entire point of the change.

It also finishes §2(b): a seat that is down now buys the enemy nothing. §2(b) fixed
the head-count half; a corpse still carried its full weight until this.

Measured, 640 runs/arm, same grid:

  solo                    69.0%   (unchanged — corpus intact)
  + Pete                  76.8%   (+7.8pp)
  + a human cleric peer   77.6%   (+8.6pp)

  band                 solo    +Pete     lift
  trailing (<40%)     10.0%    31.0%   +21.0pp
  middle              58.9%    76.8%   +17.9pp
  leading (>=70%)     93.5%    99.2%    +5.7pp

Help, never a carry: he rescues the players who were drowning and barely moves the
ones who were already fine — and he stays below a real human of the leader's level,
which is the invariant a hireling must never break.

Claude-Session: https://claude.ai/code/session_01J5SQZWoLmL3M3mw2XmHHdy

internal/plugin/zone_combat_party.go

@@ -63,6 +63,10 @@ func (p *AdventurePlugin) runZoneCombatRoster(
 		builds  []seatBuild
 		seated  []id.UserID
 		enemy   Combatant
+		// Parallel to players: what each seat is worth, priced once the roster is
+		// final. A member who sat the encounter out is in none of these.
+		levels     []int
+		companions []bool
 	)
 
 	for i, uid := range roster {
@@ -93,6 +97,7 @@ func (p *AdventurePlugin) runZoneCombatRoster(
 			players = append(players, player)
 			builds = append(builds, seatBuild{uid: uid, mods: player.Mods, companion: true})
 			seated = append(seated, uid)
+			levels, companions = append(levels, level), append(companions, true)
 			continue
 		}
 
@@ -154,8 +159,21 @@ func (p *AdventurePlugin) runZoneCombatRoster(
 			uid: uid, dndChar: dndChar, advChar: advChar, equip: equip, mods: player.Mods,
 		})
 		seated = append(seated, uid)
+		lvl := 0
+		if dndChar != nil {
+			lvl = dndChar.Level
+		}
+		levels, companions = append(levels, lvl), append(companions, false)
 	}
 
+	// What each seat costs the enemy, priced against the leader — over the seats
+	// that were actually seated, so a member left behind is not charged.
+	ptrs := make([]*Combatant, len(players))
+	for i := range players {
+		ptrs[i] = &players[i]
+	}
+	applySeatWeights(ptrs, levels, companions)
+
 	res := simulateParty(players, enemy, phases)
 	dumpCombatEventsIfDebug(
 		fmt.Sprintf("zone:%s vs %s (%d seat(s))", monster.ID, players[0].Name, len(players)),