J3 D11: T5 difficulty lift (leaders-define-band) + empty-EnemyID combat guard

Boss-only tuning at the L15/L16 mid-range (D8-f #2 had tested T5 at the L12
floor where everything walls). The two T5 zones needed opposite treatment:

- dragons_lair (infernax): impossible wall, leaders 0-2% -> HP 546->405,
  AC 22->20, frightful-presence stun 0.80->0.40, multiattack 49->42.
- abyss_portal (belaxath): leader faceroll 88-92% -> HP 262->300, AC 19->20,
  multiattack 40->41.

Final n=50: leaders 61% at L15 (both zones), 72-79% at L16 -- same
leaders-define-band shape as T4; casters ~0% (J3 caster-track gap, not
monster-tunable). Bosses are the sole binding lever (every run decided at the
boss; standard/elite pools already survivable).

Also harden handleFightCmd: a malformed bestiary entry with an empty ID was
silently persisting an enemy-less combat session that spun to autoDriveCombat's
200-round cap. Now treated as a bestiary miss (fail loud). Writeup:
sim_results/t5_findings.md (gitignored).

internal/plugin/combat_cmd.go

@@ -80,7 +80,12 @@ func (p *AdventurePlugin) handleFightCmd(ctx MessageContext) error {
 	} else {
 		monster, ok = pickZoneEnemy(zone, run.RunID, run.CurrentRoom, true)
 	}
-	if !ok {
+	if !ok || monster.ID == "" {
+		// monster.ID == "" guards a malformed bestiary entry (e.g. one whose
+		// ID field was dropped): startCombatSession would otherwise persist a
+		// session with an empty EnemyID, and the turn engine — having no enemy
+		// to resolve — spins inertly until autoDriveCombat's round cap. Fail
+		// loudly instead of stalling.
 		return p.replyDM(ctx, "_(No bestiary entry for this encounter — file a bug. `!zone abandon` to bail.)_")
 	}