N3/P8: scale the enemy's action economy to the party

A party of N used to face one enemy swing a round against a single seat, so
each member absorbed ~1/N² of the solo incoming and cleared 100% of every T5
cell. The enemy now takes enemyActionsThisRound() attack-actions, each
re-targeted at a fresh standing seat, in both combat engines:

  - auto-resolve (simulatePartyRound): enemyRoundSwings loops the actions,
    re-rolling each target's pet procs.
  - turn engine (stepEnemyTurn): enemyAttackAction resolves one full SRD
    multiattack per action; step() no longer blanket-stamps the enemy turn to
    one seat, since a party's enemy now hits several.

The action count is a fractional expectation realised as a per-round coin-flip
(2.4 for a duo, 2N-1 for N>=3), because the integer lever is too coarse at small
N -- against a duo, 2 actions is a ~91% faceroll and 3 a ~45% grinder, with
nothing between. A light party-only enemy HP scalar (x1.15) trims the martial
ceiling that action count alone leaves at 100%.

Solo is exempt on both levers (1 action, no RNG draw; x1.0 HP), so
TestCombatCharacterization is byte-identical and the d8prereq corpus still
compares. Sim band (party of 3, T5, HP scaled): fighter 70->90%, cleric-led
27->72% -- monotonic by party size, no composition worse than soloing.

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

internal/plugin/combat_session.go

@@ -488,7 +488,10 @@ func (p *AdventurePlugin) startPartyCombatSession(
 	if len(seats) == 0 {
 		return nil, fmt.Errorf("start combat session: empty roster")
 	}
-	enemyHP := enemy.Stats.MaxHP
+	// Party-only enemy HP bump (solo roster scales by 1.0). The per-turn rebuild
+	// in partyCombatantsForSession applies the identical scalar to the enemy's
+	// Stats.MaxHP, so the persisted current HP and the rebuilt max never drift.
+	enemyHP := scaledEnemyMaxHP(enemy.Stats.MaxHP, len(seats))
 	owner := seats[0]
 	sess, err := startCombatSession(owner.UserID, runID, encounterID, enemyID,
 		owner.HP, owner.HPMax, enemyHP, enemyHP)