Death source tracking + combat threat/jitter fixes

- Adventure characters now record DeathSource ("adventure"|"arena") and
  DeathLocation when killed. Threaded through Kill() and transitionDeath.
  Daily report uses the recorded death location instead of misattributing
  arena deaths to the day's adventure log: when DeathSource != "adventure",
  the adventure block shows the alive icon and a separate "Later fell in
  {location}." line. Standout-loss flavor uses DeathLocation. Empty
  DeathSource (legacy rows) falls back to current behavior.
- calcDamage applies a ±15% per-hit jitter, so successive hits in a phase
  no longer produce identical numbers. Previously every hit in a phase was
  flat (e.g. 17, 17, 17, 17) and mirror-symmetric between player/enemy,
  reading as scripted.
- assessThreat rewritten to use the engine's actual penetration formula:
  damage-per-round each direction, then rounds-to-kill ratio. The old
  additive HP+Attack*3 model ignored Defense, so even fights could be
  rated trivial — players died after consumables were skipped with the
  "threat assessed as manageable" message.
- SelectConsumables never skips on trivial when arenaRound > 0. Arena
  losses cost real money and equipment, so the cost of a wrong assessment
  is too high to gamble on; adventure-side fights still skip trivial
  threats to avoid wasting items on chump enemies.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/plugin/combat_engine.go

@@ -644,12 +644,17 @@ func applyAbility(st *combatState, player, enemy *Combatant, phase *CombatPhase,
 // reduction rather than flat subtraction, so damage is never fully negated.
 // Formula: rawAtk * reduction where reduction = K / (K + effectiveDef).
 // K=40 means 40 defense halves incoming damage; 80 defense reduces by 67%.
+//
+// A ±15% per-hit jitter is applied so successive hits in the same phase don't
+// produce identical numbers — the previous flat-damage output read as scripted.
 func calcDamage(attack int, atkWeight, dmgBonus float64, defense int, defWeight, dmgReduct float64) int {
 	const K = 40.0
 	rawAtk := float64(attack) * atkWeight * (1 + dmgBonus)
 	effectiveDef := float64(defense) * defWeight * dmgReduct
 	reduction := K / (K + effectiveDef)
 	dmg := rawAtk * reduction
+	jitter := 0.85 + rand.Float64()*0.30 // 0.85 .. 1.15
+	dmg *= jitter
 	if dmg < 1 {
 		return 1
 	}