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/combat_party_start.go

@@ -58,6 +58,10 @@ func (p *AdventurePlugin) buildFightSeats(
 			senderSkip = why
 		}
 	}
+	// Parallel to `seats`, so a skipped member leaves no gap: what a seat costs the
+	// enemy is priced from these once the roster is final.
+	var levels []int
+	var companions []bool
 	for i, uid := range roster {
 		leader := i == 0
 
@@ -84,6 +88,7 @@ func (p *AdventurePlugin) buildFightSeats(
 				HPMax:  player.Stats.MaxHP,
 				Mods:   player.Mods, C: &player, EngineDriven: true,
 			})
+			levels, companions = append(levels, level), append(companions, true)
 			continue
 		}
 
@@ -122,7 +127,7 @@ func (p *AdventurePlugin) buildFightSeats(
 			trySimAutoArm(c)
 			armed = consumeArmedAbility(c)
 		}
-		player, e, _, err := p.buildZoneCombatants(uid, monster, tier, dmMood, armed)
+		player, e, dc, err := p.buildZoneCombatants(uid, monster, tier, dmMood, armed)
 		if err != nil {
 			if leader {
 				return nil, nil, "", "Couldn't set up the fight: " + err.Error()
@@ -141,7 +146,17 @@ func (p *AdventurePlugin) buildFightSeats(
 		seats = append(seats, CombatSeatSetup{
 			UserID: uid, HP: hp, HPMax: hpMax, Mods: player.Mods, C: &player, ArmedAbility: armed,
 		})
+		lvl := 0
+		if dc != nil {
+			lvl = dc.Level
+		}
+		levels, companions = append(levels, lvl), append(companions, false)
 	}
+
+	// Price each seat against the leader. It runs here, over the seats that were
+	// actually seated — a member who was skipped (downed, busy elsewhere) never
+	// joined the fight and must not be charged to the enemy.
+	applySeatWeights(seatCombatants(seats), levels, companions)
 	return seats, enemy, senderSkip, ""
 }